Clinical Trials MCP Server

https://clinicaltrials.gov/find-studies/constructing-complex-search-queries Title: Constructing Complex Search Queries | ClinicalTrials.gov URL Source: https://clinicaltrials.gov/find-studies/constructing-complex-search-queries Markdown Content: Constructing Complex Search Queries | ClinicalTrials.gov =============== Hide glossary #### Glossary Study record managers: refer to the [Data Element Definitions](https://clinicaltrials.gov/policy#data-element-definitions) if submitting registration or results information. Search for terms * Accepts healthy volunteers A type of eligibility criteria that indicates whether people who do not have the condition/disease being studied can participate in that clinical study. * Active comparator arm An arm type in which a group of participants receives an intervention/treatment considered to be effective (or active) by health care providers. * Adverse event An unfavorable change in the health of a participant, including abnormal laboratory findings, that happens during a clinical study or within a certain amount of time after the study has ended. This change may or may not be caused by the intervention/treatment being studied. * Age or age group A type of eligibility criteria that indicates the age a person must be to participate in a clinical study. This may be indicated by a specific age or the following age groups: The age groups are: * Child (birth-17) * Adult (18-64) * Older Adult (65+) * All-cause mortality A measure of all deaths, due to any cause, that occur during a clinical study. * Allocation A method used to assign participants to an arm of a clinical study. The types of allocation are randomized allocation and nonrandomized. * Arm A group or subgroup of participants in a clinical trial that receives a specific intervention/treatment, or no intervention, according to the trial's protocol. * Arm type A general description of the clinical trial arm. It identifies the role of the intervention that participants receive. Types of arms include experimental arm, active comparator arm, placebo comparator arm, sham comparator arm, and no intervention arm. * Baseline characteristics Data collected at the beginning of a clinical study for all participants and for each arm or comparison group. These data include demographics, such as age, sex/gender, race and ethnicity, and study-specific measures (for example, systolic blood pressure, prior antidepressant treatment). * Canceled submission Indicates that the study sponsor or investigator recalled a submission of study results before quality control (QC) review took place. If the submission was canceled on or after May 8, 2018, the date is shown. After submission of study results, a study record cannot be modified until QC review is completed, unless the submission is canceled. * Certain agreements Information required by the Food and Drug Administration Amendments Act of 2007. In general, this is a description of any agreement between the sponsor of a clinical study and the principal investigator (PI) that does not allow the PI to discuss the results of the study or publish the study results in a scientific or academic journal after the study is completed. * Certification A sponsor or investigator may submit a certification to delay submission of results information if they are applying for FDA approval of a new drug or device, or new use of an already approved drug or device. A sponsor or investigator who submits a certification can delay results submission up to 2 years after the certification/extension first submitted date, unless certain events occur sooner. See [Delay Results Type](https://clinicaltrials.gov/policy/results-definitions#DelayResultsType) in the Results Data Element definitions for more information about this certification. * Certification/extension first posted The date on which information about a certification to delay submission of results or an extension request was first available on ClinicalTrials.gov. ClinicalTrials.gov does not indicate whether the submission was a certification or extension request. There is typically a delay between the date the study sponsor or investigator submitted the certification or extension request and the first posted date. * Certification/extension first submitted The date on which the study sponsor or investigator first submitted a certification or an extension request to delay submission of results. A sponsor or investigator who submits a certification can delay results submission up to 2 years after this date, unless certain events occur sooner. There is typically a delay between the date the certification or extension request was submitted and the date the information is first available on ClinicalTrials.gov (certification/extension first posted). * Certification/extension first submitted that met QC criteria The date on which the study sponsor or investigator first submitted a certification or an extension request that is consistent with National Library of Medicine (NLM) quality control (QC) review criteria. The sponsor or investigator may need to revise and submit a certification or extension request one or more times before NLM's QC review criteria are met. It is the responsibility of the sponsor or investigator to ensure that the study record is consistent with the NLM QC review criteria. Meeting QC criteria for an extension request does not mean that the National Institutes of Health (NIH) has determined that the request demonstrates good cause. The process for review and granting of extension requests by the NIH is being developed. * Clinical study A research study involving human volunteers (also called participants) that is intended to add to medical knowledge. There are two types of clinical studies: interventional studies (also called clinical trials) and observational studies. * Clinical trial Another name for an interventional study. * ClinicalTrials.gov identifier (NCT number)The unique identification code given to each clinical study upon registration at ClinicalTrials.gov. The format is "NCT" followed by an 8-digit number (for example, NCT00000419). * Collaborator An organization other than the sponsor that provides support for a clinical study. This support may include activities related to funding, design, implementation, data analysis, or reporting. * Condition/disease The disease, disorder, syndrome, illness, or injury that is being studied. On ClinicalTrials.gov, conditions may also include other health-related issues, such as lifespan, quality of life, and health risks. * Contact The name and contact information for the person who can answer enrollment questions for a clinical study. Each location where the study is being conducted may also have a specific contact, who may be better able to answer those questions. * Cross-over assignment A type of intervention model describing a clinical trial in which groups of participants receive two or more interventions in a specific order. For example, two-by-two cross-over assignment involves two groups of participants. One group receives drug A during the initial phase of the trial, followed by drug B during a later phase. The other group receives drug B during the initial phase, followed by drug A. So during the trial, participants "cross over" to the other drug. All participants receive drug A and drug B at some point during the trial but in a different order, depending on the group to which they are assigned. * Data Monitoring Committee (DMC)A group of independent scientists who monitor the safety and scientific integrity of a clinical trial. The DMC can recommend to the sponsor that the trial be stopped if it is not effective, is harming participants, or is unlikely to serve its scientific purpose. Members are chosen based on the scientific skills and knowledge needed to monitor the particular trial. Also called a data safety and monitoring board, or DSMB. * Early Phase 1 (formerly listed as Phase 0)A phase of research used to describe exploratory trials conducted before traditional phase 1 trials to investigate how or whether a drug affects the body. They involve very limited human exposure to the drug and have no therapeutic or diagnostic goals (for example, screening studies, microdose studies). * Eligibility criteria The key requirements that people who want to participate in a clinical study must meet or the characteristics they must have. Eligibility criteria consist of both inclusion criteria (which are required for a person to participate in the study) and exclusion criteria (which prevent a person from participating). Types of eligibility criteria include whether a study accepts healthy volunteers, has age or age group requirements, or is limited by sex. * Enrollment The number of participants in a clinical study. The "estimated" enrollment is the target number of participants that the researchers need for the study. * Exclusion criteria A type of eligibility criteria. These are reasons that a person is not allowed to participate in a clinical study. * Expanded access A way for patients with serious diseases or conditions who cannot participate in a clinical trial to gain access to a medical product that has not been approved by the U.S. Food and Drug Administration (FDA). Also called compassionate use. There are different expanded access types. For more information, see FDA [Expanded Access: Information for Patients](https://www.fda.gov/news-events/public-health-focus/expanded-access "Expanded Access: Information for Patients (opens in a new tab)"). * Expanded access status * **Available:**Expanded access is currently available for this investigational treatment, and patients who are not participants in the clinical study may be able to gain access to the drug, biologic, or medical device being studied. * **No longer available:**Expanded access was available for this intervention previously but is not currently available and will not be available in the future. * **Temporarily not available:**Expanded access is not currently available for this intervention but is expected to be available in the future. * **Approved for marketing:** The intervention has been approved by the U.S. Food and Drug Administration for use by the public. * Expanded access type Describes the category of expanded access under U.S. Food and Drug Administration (FDA) regulations. There are three types of expanded access: * **Individual Patients**: Allows a single patient, with a serious disease or condition who cannot participate in a clinical trial, access to a drug or biological product that has not been approved by the FDA. This category also includes access in an emergency situation. * **Intermediate-size Population**: Allows more than one patient (but generally fewer patients than through a Treatment IND/Protocol) access to a drug or biological product that has not been approved by the FDA. This type of expanded access is used when multiple patients with the same disease or condition seek access to a specific drug or biological product that has not been approved by the FDA. * **[](https://clinicaltrials.gov/)Treatment IND/Protocol**: Allows a large, widespread population access to a drug or biological product that has not been approved by the FDA. This type of expanded access can only be provided if the product is already being developed for marketing for the same use as the expanded access use. * Experimental arm An arm type in which a group of participants receives the intervention/treatment that is the focus of the clinical trial. * Extension request In certain circumstances, a sponsor or investigator may request an extension to delay the standard results submission deadline (generally one year after the primary completion date). The request for an extension must demonstrate good cause (for example, the need to preserve the scientific integrity of an ongoing masked trial). All requests must be reviewed and granted by the National Institutes of Health. This process for review and granting of extension requests is being developed. See [Delay Results Type](https://clinicaltrials.gov/policy/results-definitions#DelayResultsType) in the Results Data Element definitions for more information. * Facility name The name of the hospital or institution where a clinical study takes place. Note that not all study records include this information. To use this filter, you can enter some or all of a facility name, or type a few letters and select from the list that appears. * Factorial assignment A type of intervention model describing a clinical trial in which groups of participants receive one of several combinations of interventions. For example, two-by-two factorial assignment involves four groups of participants. Each group receives one of the following pairs of interventions: (1) drug A and drug B, (2) drug A and a placebo, (3) a placebo and drug B, or (4) a placebo and a placebo. So during the trial, all possible combinations of the two drugs (A and B) and the placebos are given to different groups of participants. * FDAAA 801 Violations A FDAAA 801 Violation is shown on a study record when the U.S. Food and Drug Administration (FDA) has issued a Notice of Noncompliance to the responsible party of an applicable clinical trial. A Notice of Noncompliance indicates that the FDA has determined the responsible party was not in compliance with the registration or results reporting requirements for the clinical trial under the Food and Drug Administration Amendments Act of 2007, Section 801 (FDAAA 801). The National Library of Medicine (NLM) is required by FDAAA 801 to add information to a study record about any FDAAA 801 Violation. This information is provided by the FDA. There are three categories of information that may be included: * Violation: Shown when the FDA issues a Notice of Noncompliance and posts the Notice of Noncompliance on its designated webpage. There are three types of violations: * Failure to submit required clinical trial information * Submission of false or misleading clinical trial information * Failure to submit primary and secondary outcomes * Correction: Shown when the FDA confirms that the responsible party has updated the study record to correct the violation and posts the correction notice on its designated webpage. Because of the time for FDA review and processing, there may be a delay between the date when the study record was updated and the addition of correction information to the FDAAA 801 Violation information. * Penalty: Shown when the FDA imposes a penalty for the violation and posts the penalty notice on its designated webpage. * First posted The date on which the study record was first available on ClinicalTrials.gov after National Library of Medicine (NLM) [quality control (QC) review](https://clinicaltrials.gov/submit-studies/prs-help/protocol-registration-quality-control-review-criteria) has concluded. There is typically a delay of a few days between the date the study sponsor or investigator submitted the study record and the first posted date. * First submitted The date on which the study sponsor or investigator first submitted a study record to ClinicalTrials.gov. There is typically a delay of a few days between the first submitted date and the record's availability on ClinicalTrials.gov (the first posted date). * First submitted that met QC criteria The date on which the study sponsor or investigator first submits a study record that is consistent with National Library of Medicine (NLM) quality control (QC) review criteria. The sponsor or investigator may need to revise and submit a study record one or more times before NLM's QC review criteria are met. It is the responsibility of the sponsor or investigator to ensure that the study record is consistent with the NLM QC review criteria. * Food and Drug Administration Amendments Act of 2007, Section 801 (FDAAA 801)U.S. Public Law 110-85, which was enacted on September 27, 2007. Section 801 of FDAAA amends Section 402 of the U.S. Public Health Service Act to expand ClinicalTrials.gov and create a clinical study results database. For more information on FDAAA 801, see the [Clinical Trial Reporting Requirements](https://clinicaltrials.gov/policy/reporting-requirements#final-rule) page on this site. * Funder type Describes the organization that provides funding or support for a clinical study. This support may include activities related to funding, design, implementation, data analysis, or reporting. Organizations listed as sponsors and collaborators for a study are considered the funders of the study. ClinicalTrials.gov refers to four types of funders: * U.S. National Institutes of Health * Other U.S. Federal agencies (for example, Food and Drug Administration, Centers for Disease Control and Prevention, or U.S. Department of Veterans Affairs) * Industry (for example: pharmaceutical and device companies) * All others (including individuals, universities, and community-based organizations) * Gender-based eligibility A type of eligibility criteria that indicates whether eligibility to participate in a clinical study is based on a person's self-representation of gender identity. Gender identity refers to a person's own sense of gender, which may or may not be the same as their biological sex. * Group/cohort A group or subgroup of participants in an observational study that is assessed for biomedical or health outcomes. * Human subjects protection review board A group of people who review, approve, and monitor the clinical study's protocol. Their role is to protect the rights and welfare of people participating in a study (referred to as human research subjects), such as reviewing the informed consent form. The group typically includes people with varying backgrounds, including a community member, to make sure that research activities conducted by an organization are completely and adequately reviewed. Also called an institutional review board, or IRB, or an ethics committee. For more information, see [Who can join clinical research?](https://clinicaltrials.gov/study-basics/learn-about-studies#q3) on this site. * Inclusion criteria A type of eligibility criteria. These are the reasons that a person is allowed to participate in a clinical study. * Informed consent A process used by researchers to communicate to potential and enrolled participants the risks and potential benefits of participating in a clinical study. For more information, see [Who can join clinical research?](https://clinicaltrials.gov/study-basics/learn-about-studies#q3) on this site. * Informed consent form (ICF)The document used in the informed consent or process. * Intervention model The general design of the strategy for assigning interventions to participants in a clinical study. Types of intervention models include: single group assignment, parallel assignment, cross-over assignment, and factorial assignment. * Intervention/treatment A process or action that is the focus of a clinical study. Interventions include drugs, medical devices, procedures, vaccines, and other products that are either investigational or already available. Interventions can also include noninvasive approaches, such as education or modifying diet and exercise. * Interventional study (clinical trial)A type of clinical study in which participants are assigned to groups that receive one or more intervention/treatment (or no intervention) so that researchers can evaluate the effects of the interventions on biomedical or health-related outcomes. The assignments are determined by the study's protocol. Participants may receive diagnostic, therapeutic, or other types of interventions. * Investigator A researcher involved in a clinical study. Related terms include site principal investigator, site sub-investigator, study chair, study director, and study principal investigator. * Last update posted The most recent date on which changes to a study record were made available on ClinicalTrials.gov. There may be a delay between when the changes were submitted to ClinicalTrials.gov by the study's sponsor or investigator (the last update submitted date) and the last update posted date. * Last update submitted The most recent date on which the study sponsor or investigator submitted changes to a study record to ClinicalTrials.gov. There is typically a delay of a few days between the last update submitted date and when the date changes are posted on ClinicalTrials.gov (the last update posted date). * Last update submitted that met QC criteria The most recent date on which the study sponsor or investigator submitted changes to a study record that are consistent with National Library of Medicine (NLM) quality control (QC) review criteria. It is the responsibility of the sponsor or investigator to ensure that the study record is consistent with the NLM QC review criteria. * Last verified The most recent date on which the study sponsor or investigator confirmed the information about a clinical study on ClinicalTrials.gov as accurate and current. If a study with a recruitment status of recruiting; not yet recruiting; or active, not recruiting has not been confirmed within the past 2 years, the study's recruitment status is shown as unknown. * Location A place where a research site for a clinical study can be found. Location information can be searched using a facility name, a city, state, zip code, or country. A location where a study is being conducted may also include contact information. * Location countries Countries in which research facilities for a study are located. A country is listed only once, even if there is more than one facility in the country. The list includes all countries as of the last update submitted date; any country for which all facilities were removed from the study record are listed under removed location countries. * Masking A clinical trial design strategy in which one or more parties involved in the trial, such as the investigator or participants, do not know which participants have been assigned which interventions. Types of masking include: open label, single blind masking, and double-blind masking. * NCT number A unique identification code given to each clinical study record registered on ClinicalTrials.gov. The format is "NCT" followed by an 8-digit number (for example, NCT00000419). Also called the ClinicalTrials.gov identifier. * No intervention arm An arm type in which a group of participants does not receive any intervention/treatment during the clinical trial. * Observational study A type of clinical study in which participants are identified as belonging to study groups and are assessed for biomedical or health outcomes. Participants may receive diagnostic, therapeutic, or other types of interventions, but the investigator does not assign participants to a specific interventions/treatment. A patient registry is a type of observational study. * Observational study model The general design of the strategy for identifying and following up with participants during an observational study. Types of observational study models include cohort, case-control, case-only, case-cross-over, ecologic or community studies, family-based, and other. * Other adverse event An adverse event that is not a serious adverse event, meaning that it does not result in death, is not life-threatening, does not require inpatient hospitalization or extend a current hospital stay, does not result in an ongoing or significant incapacity or interfere substantially with normal life functions, and does not cause a congenital anomaly or birth defect; it also does not put the participant in danger and does not require medical or surgical intervention to prevent one of the results listed above. * Other study IDs Identifiers or ID numbers other than the NCT number that are assigned to a clinical study by the study's sponsor, funders, or others. These numbers may include unique identifiers from other trial registries and National Institutes of Health grant numbers. * Other terms In the search feature, the Other terms field is used to narrow a search. For example, you may enter the name of a drug or the NCT number of a clinical study to limit the search to study records that contain these words. * Outcome measure For clinical trials, a planned measurement described in the protocol that is used to determine the effect of an intervention/treatment on participants. For observational studies, a measurement or observation that is used to describe patterns of diseases or traits, or associations with exposures, risk factors, or treatment. Types of outcome measures include primary outcome measure and secondary outcome measure. * Parallel assignment A type of intervention model describing a clinical trial in which two or more groups of participants receive different interventions. For example, a two-arm parallel assignment involves two groups of participants. One group receives drug A, and the other group receives drug B. So during the trial, participants in one group receive drug A "in parallel" to participants in the other group, who receive drug B. * Participant flow A summary of the progress of participants through each stage of a clinical study, by study arm or group/cohort. This includes the number of participants who started, completed, and dropped out of the study. * Patient registry A type of observational study that collects information about patients' medical conditions and/or treatments to better understand how a condition or treatment affects patients in the real world. * Phase The stage of a clinical trial studying a drug or biological product, based on definitions developed by the U.S. Food and Drug Administration (FDA). The phase is based on the study's objective, the number of participants, and other characteristics. There are five phases: Early Phase 1 (formerly listed as Phase 0), Phase 1, Phase 2, Phase 3, and Phase 4. Not Applicable is used to describe trials without FDA-defined phases, including trials of devices or behavioral interventions. * Phase 1 A phase of research to describe clinical trials that focus on the safety of a drug. They are usually conducted with healthy volunteers, and the goal is to determine the drug's most frequent and serious adverse events and, often, how the drug is broken down and excreted by the body. These trials usually involve a small number of participants. * Phase 2 A phase of research to describe clinical trials that gather preliminary data on whether a drug works in people who have a certain condition/disease (that is, the drug's effectiveness). For example, participants receiving the drug may be compared to similar participants receiving a different treatment, usually an inactive substance (called a placebo) or a different drug. Safety continues to be evaluated, and short-term adverse events are studied. * Phase 3 A phase of research to describe clinical trials that gather more information about a drug's safety and effectiveness by studying different populations and different dosages and by using the drug in combination with other drugs. These studies typically involve more participants. * Phase 4 A phase of research to describe clinical trials occurring after FDA has approved a drug for marketing. They include postmarket requirement and commitment studies that are required of or agreed to by the study sponsor. These trials gather additional information about a drug's safety, efficacy, or optimal use. * Phase Not Applicable Describes trials without FDA-defined phases, including trials of devices or behavioral interventions. * Placebo An inactive substance or treatment that looks the same as, and is given in the same way as, an active drug or intervention/treatment being studied. * Placebo comparator arm An arm type in which a group of participants receives a placebo during a clinical trial. * Primary completion date The date on which the last participant in a clinical study was examined or received an intervention to collect final data for the primary outcome measure. Whether the clinical study ended according to the protocol or was terminated does not affect this date. For clinical studies with more than one primary outcome measure with different completion dates, this term refers to the date on which data collection is completed for all the primary outcome measures. The "estimated" primary completion date is the date that the researchers think will be the primary completion date for the study. * Primary outcome measure In a clinical study's protocol, the planned outcome measure that is the most important for evaluating the effect of an intervention/treatment. Most clinical studies have one primary outcome measure, but some have more than one. * Primary purpose The main reason for the clinical trial. The types of primary purpose are: treatment, prevention, diagnostic, supportive care, screening, health services research, basic science, and other. * Principal investigator (PI)The person who is responsible for the scientific and technical direction of the entire clinical study. * Protocol The written description of a clinical study. It includes the study's objectives, design, and methods. It may also include relevant scientific background and statistical information. * Quality control (QC) review National Library of Medicine (NLM) staff perform a limited review of submitted study records for apparent errors, deficiencies, or inconsistencies. NLM staff identify potential major and advisory issues and provide comments directly to the study sponsor or investigator. Major issues identified in QC review must be addressed or corrected (see First submitted that met QC criteria and Results first submitted that met QC criteria). Advisory issues are suggestions to help improve the clarity of the record. NLM staff do not verify the scientific validity or relevance of the submitted information. The study sponsor or investigator is responsible for ensuring that the studies follow all applicable laws and regulations. * Randomized allocation A type of allocation strategy in which participants are assigned to the arms of a clinical trial by chance. * Recruitment status * **Not yet recruiting:** The study has not started recruiting participants. * **Recruiting:** The study is currently recruiting participants. * **Enrolling by invitation:** The study is selecting its participants from a population, or group of people, decided on by the researchers in advance. These studies are not open to everyone who meets the eligibility criteria but only to people in that particular population, who are specifically invited to participate. * **Active, not recruiting:** The study is ongoing, and participants are receiving an intervention or being examined, but potential participants are not currently being recruited or enrolled. * **Suspended:** The study has stopped early but may start again. * **Terminated:** The study has stopped early and will not start again. Participants are no longer being examined or treated. * **Completed:** The study has ended normally, and participants are no longer being examined or treated (that is, the last participant's last visit has occurred). * **Withdrawn:** The study stopped early, before enrolling its first participant. * **Unknown:** A study on ClinicalTrials.gov whose last known status was recruiting; not yet recruiting; or active, not recruiting but that has passed its completion date, and the status has not been last verified within the past 2 years. * Registration The process of submitting and updating summary information about a clinical study and its protocol, from its beginning to end, to a structured, public Web-based study registry that is accessible to the public, such as ClinicalTrials.gov. * Removed location countries Countries that appeared under location countries but were removed from the study record by the sponsor or investigator. * Reporting group A grouping of participants in a clinical study that is used for summarizing the data collected during the study. This grouping may be the same as or different from a study arm or group. * Responsible party The person responsible for submitting information about a clinical study to ClinicalTrials.gov and updating that information. Usually the study sponsor or investigator. * Results database A structured online system, such as the ClinicalTrials.gov results database, that provides the public with access to registration and summary results information for completed or terminated clinical studies. A study with results available on ClinicalTrials.gov is described as having the results "posted." **Note:** The ClinicalTrials.gov results database became available in September 2008. Older studies are unlikely to have results available in the database. * Results delayed Indicates that the sponsor or investigator submitted a certification or extension request. * Results first posted The date on which summary results information was first available on ClinicalTrials.gov after National Library of Medicine (NLM) [quality control (QC) review](https://clinicaltrials.gov/submit-studies/prs-help/results-quality-control-review-criteria) has concluded. There is typically a delay between the date the study sponsor or investigator first submits summary results information (the results first submitted date) and the results first posted date. Some results information may be available at an earlier date if Results First Posted with QC Comments. * Results first posted with QC comments The date on which summary results information was first available on ClinicalTrials.gov with quality control review comments from the National Library of Medicine (NLM) identifying major issues that must be addressed by the sponsor or investigator. As of January 1, 2020, initial results submissions for applicable clinical trials (ACTs) that do not meet [quality control review criteria](https://clinicaltrials.gov/submit-studies/prs-help/results-quality-control-review-criteria) will be publicly posted on ClinicalTrials.gov with brief standardized major comments. Accordingly, the Results First Posted with QC Comments date may be earlier than the Results First Posted date for an ACT with summary results information that is not consistent with NLM quality control review criteria. * Results first submitted The date on which the study sponsor or investigator first submits a study record with summary results information. There is typically a delay between the results first submitted date and when summary results information becomes available on ClinicalTrials.gov (the results first posted date). * Results first submitted that met QC criteria The date on which the study sponsor or investigator first submits a study record with summary results information that is consistent with National Library of Medicine (NLM) quality control (QC) review criteria. The sponsor or investigator may need to revise and submit results information one or more times before NLM's QC review criteria are met. It is the responsibility of the sponsor or investigator to ensure that the study record is consistent with the NLM QC review criteria. * Results returned after quality control review The date on which the National Library of Medicine provided quality control (QC) review comments to the study sponsor or investigator. The sponsor or investigator must address major issues identified in the review comments. If there is a date listed for results returned after quality control review, but there is not a subsequent date listed for results submitted to ClinicalTrials.gov, this means that the submission is pending changes by the sponsor or investigator. * Results submitted to ClinicalTrials.gov Indicates that the study sponsor or investigator has submitted summary results information for a clinical study to ClinicalTrials.gov but the quality control (QC) review process has not concluded. The results submitted date indicates when the study sponsor or investigator first submitted summary results information or submitted changes to summary results information. Submissions with changes are typically in response to QC review comments from the National Library of Medicine (NLM). If there is a date listed for results submitted to ClinicalTrials.gov, but there is not a subsequent date listed for results returned after quality control review, this means that the submission is pending review by NLM. * Secondary outcome measure In a clinical study's protocol, a planned outcome measure that is not as important as the primary outcome measure for evaluating the effect of an intervention but is still of interest. Most clinical studies have more than one secondary outcome measure. * Serious adverse event An adverse event that results in death, is life-threatening, requires inpatient hospitalization or extends a current hospital stay, results in an ongoing or significant incapacity or interferes substantially with normal life functions, or causes a congenital anomaly or birth defect. Medical events that do not result in death, are not life-threatening, or do not require hospitalization may be considered serious adverse events if they put the participant in danger or require medical or surgical intervention to prevent one of the results listed above. * Sex A type of eligibility criteria that indicates the sex of people who may participate in a clinical study (all, female, male). Sex is a person's classification as female or male based on biological distinctions. Sex is distinct from gender-based eligibility. * Sham comparator arm An arm type in which a group of participants receives a procedure or device that appears to be the same as the actual procedure or device being studied but does not contain active processes or components. * Single group assignment A type of intervention model describing a clinical trial in which all participants receive the same intervention/treatment. * Sort studies by The Sort studies by option is used to change the order of studies listed on the Search Results page. You can sort by Relevance or Newest First: * Relevance: Studies that best match your search terms appear higher in the search results list. This is the default display for all searches. * Newest First: Studies with the most recent First posted dates appear higher in the search results list. * Sponsor The organization or person who initiates the study and who has authority and control over the study. * Statistical analysis plan (SAP)The written description of the statistical considerations and methods for analyzing the data collected in the clinical study. * Status Indicates the current recruitment status or the expanded access status. * Study completion date The date on which the last participant in a clinical study was examined or received an intervention/treatment to collect final data for the primary outcome measures, secondary outcome measures, and adverse events (that is, the last participant's last visit). The "estimated" study completion date is the date that the researchers think will be the study completion date. * Study design The investigative methods and strategies used in the clinical study. * Study documents Refers to the type of documents that the study sponsor or principal investigator may add to their study record. These include a study protocol, statistical analysis plan, and informed consent form. * Study IDs Identifiers that are assigned to a clinical study by the study's sponsor, funders, or others. They include unique identifiers from other trial study registries and National Institutes of Health grant numbers. Note: ClinicalTrials.gov assigns a unique identification code to each clinical study registered on ClinicalTrials.gov. Also called the NCT number, the format is "NCT" followed by an 8-digit number (for example, NCT00000419). * Study record An entry on ClinicalTrials.gov that contains a summary of a clinical study's protocol information, including the recruitment status; eligibility criteria; contact information; and, in some cases, summary results. Each study record is assigned a ClinicalTrials.gov identifier, or NCT number. * Study registry A structured online system, such as ClinicalTrials.gov, that provides the public with access to summary information about ongoing and completed clinical studies. * Study results A study record that includes the summary results posted in the ClinicalTrials.gov results database. Summary results information includes participant flow, baseline characteristics, outcome measures, and adverse events (including serious adverse events). * Study start date The actual date on which the first participant was enrolled in a clinical study. The "estimated" study start date is the date that the researchers think will be the study start date. * Study type Describes the nature of a clinical study. Study types include interventional studies (also called clinical trials), observational studies (including patient registries), and expanded access. * Submitted date The date on which the study sponsor or investigator submitted a study record that is consistent with National Library of Medicine (NLM) quality control (QC) review criteria. * Title The official title of a protocol used to identify a clinical study or a short title written in language intended for the lay public. * Title acronym The acronym or initials used to identify a clinical study (not all studies have one). For example, the title acronym for the Women's Health Initiative is "WHI." * Type of intervention A process or action that is the focus of a clinical study. Interventions include drugs, medical devices, procedures, vaccines, and other products that are either investigational or already available. Interventions can also include noninvasive approaches, such as education or modifying diet and exercise. * U.S. Agency for Healthcare Research and Quality (AHRQ)An agency within the U.S. Department of Health and Human Services. AHRQ's mission is to produce evidence to make health care safer, higher quality, more accessible, equitable, and affordable, and to work within the U.S. Department of Health and Human Services and with other partners to make sure that the evidence is understood and used. * U.S. Food and Drug Administration (FDA)An agency within the U.S. Department of Health and Human Services. The FDA is responsible for protecting the public health by making sure that human and veterinary drugs, vaccines and other biological products, medical devices, the Nation's food supply, cosmetics, dietary supplements, and products that give off radiation are safe, effective, and secure. * Unknown A type of recruitment status. It identifies a study on ClinicalTrials.gov whose last known status was recruiting; not yet recruiting; or active, not recruiting but that has passed its completion date, and the status has not been verified within the past 2 years. Studies with an unknown status are considered closed studies.  An official website of the United States government Here’s how you know Here’s how you know  **Official websites use .gov** A **.gov** website belongs to an official government organization in the United States.  **Secure .gov websites use HTTPS** A **lock** ( ) or **https://** means you’ve safely connected to the .gov website. 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[Skip to main page content](https://clinicaltrials.gov/#main-content) [](https://ncbi.nlm.nih.gov/ "NIH National Library of Medicine, National Center for Biotechnology Information (opens in a new tab)") PRS Login [](https://clinicaltrials.gov/ "ClinicalTrials.gov return to the CTG homepage")[](https://clinicaltrials.gov/) Menu  * Find Studies * [Expert Search](https://clinicaltrials.gov/expert-search) * [How to Search](https://clinicaltrials.gov/find-studies/how-to-search) * [How to Use Search Results](https://clinicaltrials.gov/find-studies/how-to-use-search-results) * [How to Search for Studies with Results](https://clinicaltrials.gov/find-studies/how-to-search-for-studies-with-results) * [Constructing Complex Search Queries](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries) * [RSS Feeds](https://clinicaltrials.gov/find-studies/rss) * Study Basics * [Learn About Studies](https://clinicaltrials.gov/study-basics/learn-about-studies) * [How to Read a Study Record](https://clinicaltrials.gov/study-basics/how-to-read-study-record) * [How to Read Study Results](https://clinicaltrials.gov/study-basics/how-to-read-study-results) * [Glossary](https://clinicaltrials.gov/study-basics/glossary) * [Patient Resources](https://clinicaltrials.gov/study-basics/patient-resources) * Submit Studies * [PRS Accounts](https://clinicaltrials.gov/submit-studies/prs-accounts) * [PRS Help Resources](https://clinicaltrials.gov/submit-studies/prs-help) * Data and API * [ClinicalTrials.gov API](https://clinicaltrials.gov/data-api/api) * [About the API](https://clinicaltrials.gov/data-api/about-api) * [How to Download Study Records](https://clinicaltrials.gov/data-api/how-download-study-records) * [Access Data in FHIR](https://clinicaltrials.gov/data-api/fhir) * Policy * [FAQs](https://clinicaltrials.gov/policy/faq) * [Clinical Trial Reporting Requirements](https://clinicaltrials.gov/policy/reporting-requirements) * [FDAAA 801 and the Final Rule](https://clinicaltrials.gov/policy/fdaaa-801-final-rule) * [Protocol Registration Definitions](https://clinicaltrials.gov/policy/protocol-definitions) * [Results Definitions](https://clinicaltrials.gov/policy/results-definitions) * [Expanded Access Definitions](https://clinicaltrials.gov/policy/expanded-access-definitions) * About * [News and Updates](https://clinicaltrials.gov/about-site/news-and-updates) * [About ClinicalTrials.gov](https://clinicaltrials.gov/about-site/about-ctg) * [Trends and Charts](https://clinicaltrials.gov/about-site/trends-charts) * [Modernization](https://clinicaltrials.gov/about-site/modernization) * [Modernization Top Questions](https://clinicaltrials.gov/about-site/modernization-top-questions) * [Selected Publications](https://clinicaltrials.gov/about-site/selected-publications) * [Release Notes](https://clinicaltrials.gov/about-site/release-notes) * [Disclaimer](https://clinicaltrials.gov/about-site/disclaimer) * [Terms and Conditions](https://clinicaltrials.gov/about-site/terms-conditions) * [Linking to This Site](https://clinicaltrials.gov/about-site/linking-to) * [Accessibility](https://clinicaltrials.gov/about-site/accessibility) * My Saved Studies (0) 1. [Home](https://clinicaltrials.gov/) 2. [Find Studies](https://clinicaltrials.gov/find-studies) 3. Constructing Complex Search Queries Constructing Complex Search Queries =================================== On this page [Introduction](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#intro) [Search operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-operators) [Search terms](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-terms) [Search expressions](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-expression) Introduction ------------ This page explains the use of search operators, terms, syntax, and expressions to do an advanced search of study record data on ClinicalTrials.gov. Search expressions consist of a sequence of terms and operators. The operators affect which studies are returned and how they are ordered in retrieval sets by changing how the search terms are contextualized or interpreted. Search terms are words or phrases that must appear as values in the study records returned by the search. Users can create a search expression on the Expert Search page found in the main menu under Find Studies and on the Site Map. In addition, users can create and edit a search expression by executing a search using Basic Search and clicking the Expert Search link button in Focus Your Search. Search syntax refers to the arrangement of search terms and operators in search expressions used for evaluation by the search engine. Search operators ---------------- In general, operator precedence (from highest to lowest) is as follows: search terms and source operators, the NOT operator and context operators, the AND operator, and the OR operator. The following operator types are described in the sections below: * [Boolean operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#booleanOpr "Boolean operators")(https://clinicaltrials.gov.#booleanOpr) * [Grouping operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#groupOpr "Grouping operators")(https://clinicaltrials.gov.#groupOpr) * [Context operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#contextOpr "Context operators")(https://clinicaltrials.gov.#contextOpr) * [Source operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#sourceOpr "Source operators")(https://clinicaltrials.gov.#sourceOpr) * [Scoring operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#scoringOpr "Scoring operators")(https://clinicaltrials.gov.#scoringOpr) ### Boolean operators Boolean operators connect search terms and define their logical relationship. Operator Example Description OR youth OR teen A binary operator used to retrieve study records containing either the left or right subexpression, or both. The weighted score for determining rank order in retrieval sets will be higher when both the left and right subexpressions are present in a study record. AND heart AND attack A binary operator used to retrieve study records containing both the left and right subexpressions. NOT bethesda NOT maryland A unary operator that acts only on the right subexpression. It is used to retrieve study records that do not contain the right subexpression. ### Grouping operators Grouping operators group search terms or operators in a query. Operator Example Description """back pain"Forces a sequence of words to be treated as a phrase. ()(acetaminophen OR aspirin) AND NOT (heart failure OR heart attack)Used to increase operator precedence in a search expression. ### Context operators Context operators, which control how search terms are evaluated, are immediately followed by parameters in square brackets without any spaces. All context operators have the same precedence as the NOT operator and apply to the subexpression on their right, which immediately follows the right square bracket without a space in between. Operator Example Description Default Setting COVERAGE COVERAGE[FullMatch]pain Declares the degree to which a search term needs to match the text in an API field. There are four choices: * FullMatch—The search term must match all of the text in the field searched. * StartsWith—The search term must match the beginning of the text in the field searched. * EndsWith—The search term must match the end of the text in the field searched. * Contains—The search term must match part of the text in the field searched.Defaults to Contains when the search expression does not include a COVERAGE operator. EXPANSION *EXPANSION[None]SLE Declares the degree to which a search term may be expanded. There are five choices: * None—The term is searched for exactly as is. Case and accent marks are significant, but consecutive spaces are treated as a single space. * Term—Similar to None but includes simple lexical variants such as plurals, possessives, alternate spellings, and compound words; ignores case, hyphens, and accent marks. * Concept—Similar to Term but includes synonyms based on the Unified Medical Language System (UMLS). Also has a slight scoring penalty, ranking any records that include the search terms higher than records that include only synonyms. * Relaxation—Similar to Concept but relaxes adjacency requirements so that partial terms are matches (e.g., a search for _heart disease_ will return records with _heart_ in one place and _disease_ in another, as in the phrase "heart and lung disease"). Also has a significant scoring penalty, ranking any records that include the full search terms higher than records that include only partial terms. * Lossy—Similar to Relaxation but allows for missing partial terms (e.g., a search for _heart disease_ will return records with _heart_ but not _disease_ and records with _disease_ but not _heart_).Defaults to Relaxation when the search expression does not include an EXPANSION operator. AREA AREA[InterventionName]aspirin Declares which search area should be searched. Search areas are defined on the ClinicalTrials.gov [Search Areas](https://clinicaltrials.gov/data-api/about-api/search-areas "Search Areas")(https://clinicaltrials.gov/data-api/about-api/search-areas) page. In addition to specifying search areas, it is possible to specify a field from the [study structure](https://clinicaltrials.gov/data-api/about-api/study-data-structure "study structure")(https://clinicaltrials.gov/data-api/about-api/study-data-structure). Any field from the study structure is searchable. SEARCH **heart attack AND SEARCH[Location](AREA[LocationCity]Portland AND AREA[LocationState]Maine)Declares which subsection of the [study structure](https://clinicaltrials.gov/data-api/about-api/study-data-structure "study structure")(https://clinicaltrials.gov/data-api/about-api/study-data-structure) should be searched. For example, this operator can be used to search for a city and state within the same location (e.g. Portland, Maine), and exclude cities and states outside of that location (e.g. Portland, Oregon, and Augusta, Maine). * The EXPANSION operator interacts with other operators as follows: * Enclosing a sequence of words in quotation marks forces those words to be treated as a phrase and gives the phrase higher precedence than EXPANSION. As such, a quoted phrase following EXPANSION[Relaxation] or EXPANSION[Lossy] is effectively the same as EXPANSION[Concept]. * COVERAGE[FullMatch], COVERAGE[StartsWith], and COVERAGE[EndsWith] can be used only with EXPANSION[Concept], EXPANSION[Term], or EXPANSION[None]. ** The SEARCH operator restricts search expressions to fields within a data element so that multiple pieces from that data element can be found together. For instance, the Location data element includes LocationCity, LocationState, LocationCountry, and LocationStatus. The SEARCH[Location] operator limits the search to the Location data element. The following example uses the SEARCH[Location] operator to find site facility locations in the United States that are also recruiting participants: heart attack AND SEARCH[Location](AREA[LocationCountry]United States AND AREA[LocationStatus]Recruiting) In contrast, the following search expression, which does not use SEARCH[Location], would also return studies listing facility locations in the United States that are not recruiting and facility locations not in the United States that are recruiting: heart attack AND AREA[LocationCountry]United States AND AREA[LocationStatus]Recruiting ### Source operators Source operators find studies similar to search terms. All source operators have the same precedence as a search term. Operator Example Description MISSING AREA[ResultsFirstPostDate]MISSING Finds study records that have no values in the search area specified as a parameter. RANGE AREA[ResultsFirstPostDate]RANGE[01/01/2015, MAX]Finds study records in the search area that have a value greater than or equal to the first parameter (e.g., 01/01/2015) and less than or equal to the second parameter (e.g. MAX) in the search area. Each search area (e.g., ResultsFirstPostedDate) has a specified ordering, as follows: * Fields containing numbers and dates—Ordered in ascending sequence. * Age fields—Special ordering to accommodate units and to treat 1 year as greater than 6 months. Special RANGE operator values: * MIN—Use to indicate the smallest value of interest in the current search area. * MAX—Use to indicate the largest value of interest in the current search area. Note: The RANGE operator will not retrieve study records with no values in the search area. DISTANCE AREA[LocationGeoPoint] DISTANCE[39.0019296, -77.1044561, 25mi] AREA[LocationGeoPoint] DISTANCE[39.0019296, -77.1044561, 3.098km]Finds studies within a certain radius of a latitude and longitude. The radius must be set to at least 1 mile or 1 kilometer and cannot exceed 500 miles (805 kilometers). The range value may include decimal places. This operator is applicable to any GeoPoint data type. ALL ALL Retrieves all study records in the database. ### Scoring operators Scoring operators are used to adjust the rank order of search results by the most recent using dates or size of values. Operator Example Description TILT TILT[StudyFirstPostDate]"heart attack"Biases the scoring and rank ordering of study records in favor of the subexpression to the right by imposing a scoring penalty based on the ordering of API field values for the search area provided as a parameter (e.g., StudyFirstPostDate), with higher-ordered values having a lower penalty (e.g., more recent dates) than lower-ordered values (e.g., earlier dates). Use the TILT operator with API fields that are ordered, such as date fields. Search terms ------------ Search terms are words or phrases that must appear as values in the study records returned by the search. A search term consists of a string of characters without embedded [search operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-operators "search operators")(https://clinicaltrials.gov.#search-operators). Enclosing a multi-word term in quotation marks indicates that all of the words in the search term must be found together in that order (that is, as a phrase) for a study record to be retrieved by the search (see below).  [Enlarge image](https://clinicaltrials.gov/#search-terms-image) To search for an operator as part of a term, add a backslash before the operator (e.g., \MISSING). If the operator used is quotation marks, then only embedded quotes need to be preceded by a backslash (e.g., \"sleep deprivation"\). Search expressions ------------------ A search expression consists of sequences of search terms and operators that are evaluated by the search engine to find lists of studies. The [search operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-operators "search operators")(https://clinicaltrials.gov.#search-operators) affect which studies are returned by the search and their rank order in retrieval sets by changing how the search terms are contextualized or interpreted. All search expressions are [OR expressions](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#orExpr "OR expressions")(https://clinicaltrials.gov.#orExpr). ### Order of precedence among expressions The order in which different types of search expressions are evaluated by the search engine is presented below from highest to lowest precedence. To increase the precedence of an expression, enclose that expression in parentheses. 1. [Source expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#sourceExpr "Source expression")(https://clinicaltrials.gov.#sourceExpr). 2. [Operator expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#operatorExpr "Operator expression")(https://clinicaltrials.gov.#operatorExpr). 3. [AND expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#andExpr "AND expression")(https://clinicaltrials.gov.#andExpr). 4. [OR expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#orExpr "OR expression")(https://clinicaltrials.gov.#orExpr). ### Source expression A source expression consists of a [search term](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-terms "search term")(https://clinicaltrials.gov.#search-terms), a [range expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#rangeExpr "range expression")(https://clinicaltrials.gov.#rangeExpr), an [OR expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#orExpr "OR expression")(https://clinicaltrials.gov.#orExpr) enclosed in parentheses, and either the [MISSING](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#missingOp "MISSING")(https://clinicaltrials.gov.#missingOp) or [ALL](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#allNoneOps "ALL")(https://clinicaltrials.gov.#allNoneOps) source operator (see below). The MISSING operator can be used for Structure type fields.  [Enlarge image](https://clinicaltrials.gov/#source-expression-image) The following examples are all source expressions: * exhaustion * RANGE[21 years,MAX] or RANGE[2025-03-20, 2025-03-31] * (exhaustion OR sleep deprivation) * MISSING #### Range expression A range expression, a type of source expression, consists of a [RANGE](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#rangeOp "RANGE")(https://clinicaltrials.gov.#rangeOp) operator with minimum and maximum values as parameters. Use MIN to indicate the smallest value of interest in the search area and use MAX to indicate the largest value of interest in the search area. The parameters of a range expression may include dates, numbers, or numbers with units, such as with age. See examples above. Studies with no values in the search area will not be retrieved by a search that uses a range expression. ### Operator expression An operator expression consists of a sequence of one, or more unary operators (e.g., the [NOT](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#notOp "NOT")(https://clinicaltrials.gov.#notOp) operator and all [context operators](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#contextOpr "context operators")(https://clinicaltrials.gov.#contextOpr)), followed by a [source expression](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#sourceExpr "source expression")(https://clinicaltrials.gov.#sourceExpr). Any number of operator expressions may precede a source expression (see below).  [Enlarge image](https://clinicaltrials.gov/#operator-expression-image) The [TILT](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#tiltOp "TILT")(https://clinicaltrials.gov.#tiltOp) and [AREA](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#areaOp "AREA")(https://clinicaltrials.gov.#areaOp) operators take [search areas](https://clinicaltrials.gov/data-api/about-api/search-areas "search areas")(https://clinicaltrials.gov/data-api/about-api/search-areas) (labeled "Area" in the figure above) as a parameter. Note that the parameter is enclosed in square brackets, and there is no space between the operator and the left bracket. The [SEARCH](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#searchOp "SEARCH")(https://clinicaltrials.gov.#searchOp) operator takes either "Study" or "Location" as a parameter, as shown in the figure above. The [COVERAGE](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#coverageOp "COVERAGE")(https://clinicaltrials.gov.#coverageOp) and [EXPANSION](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#expansionOp "EXPANSION")(https://clinicaltrials.gov.#expansionOp) operators take one of a small set of choices as a parameter (shown in the figure above). The COVERAGE operator takes Contains, FullMatch, StartsWith and EndsWith. EXPANSION[Concept] and EXPANSION[Relaxation] can be used in combination with COVERAGE[Contains]. EXPANSION[Lossy] cannot be used with a COVERAGE operator. The following examples are all operator expressions that include source expressions: * **exhaustion** * NOT **exhaustion** * EXPANSION[Concept] NOT (**exhaustion OR sleep deprivation**) * heart attack AND SEARCH[Location](AREA[LocationCity] Bethesda AND AREA[LocationState] Maryland) These unary operators have the second highest precedence after source expressions. ### AND expression An AND expression consists of a list of one or more [operator expressions](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#operatorExpr "operator expressions")(https://clinicaltrials.gov.#operatorExpr), separated by one or many binary Boolean [AND](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#andOp "AND")(https://clinicaltrials.gov.#andOp) operators (see below).  [Enlarge image](https://clinicaltrials.gov/#and-expression-image) The following examples are all AND expressions that include operator expressions: * **dizziness** AND **NOT exhaustion** * **dizziness** AND **NOT exhaustio** AND **stroke** The AND operator has the second lowest precedence among search operators. All operator expressions are evaluated before the AND expressions formed by connecting the operator expressions using AND operators. ### OR expression An OR expression consists of a list of one or more [AND expressions](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#andExpr "AND expressions")(https://clinicaltrials.gov.#andExpr) (such as a [search term](https://clinicaltrials.gov/find-studies/constructing-complex-search-queries#search-terms "search term")(https://clinicaltrials.gov.#search-terms) or phrase) separated by binary Boolean OR operators (see below).  [Enlarge image](https://clinicaltrials.gov/#or-expression-image) The following examples are all OR expressions that include AND expressions: * **heart attack** OR **stroke** * **heart attack** OR **stroke** OR **dizziness AND shortness of breath** The OR operator has the lowest precedence among search operators. All AND expressions are evaluated before the OR expressions formed by connecting the AND expressions using OR operators.  National Library of Medicine 8600 Rockville Pike, Bethesda, MD 20894 [](https://ncbi.nlm.nih.gov/) ClinicalTrials.gov An official website of the [](https://clinicaltrials.gov/), [](https://clinicaltrials.gov/), [](https://clinicaltrials.gov/), and [](https://clinicaltrials.gov/). Looking for U.S. government information and services? [](https://clinicaltrials.gov/) [HHS Vulnerability Disclosure](https://www.hhs.gov/vulnerability-disclosure-policy/index.html)