Clinical trials are an essential part of the drug-development process. During any clinical trial, the sponsor of the trial creates a formal study protocol, which defines, among other things, strict inclusion and exclusion criteria that define specific demographic and medical factors that would allow potential trial participants to either be enrolled in the trial or excluded from participating in it. 

Inclusion criteria relate to specific characteristics that the potential participants must demonstrate in order to be considered for enrollment in the study. Exclusion criteria are specific factors that would disqualify a patient from being considered for enrollment in the study. Together, these parameters are often referred to as eligibility criteria.

Common inclusion and exclusion criteria

Some of the most commonly specified eligibility criteria are used to minimize confounding variables and reduce risk to the participants as much as possible.

Age: Many clinical trials specify minimum and/or maximum ages of the eligible participants. Children, adolescents and seniors are sometimes excluded, depending on the condition being studied, as they may be considered  potentially more vulnerable, due to weaker immune systems or different rates of metabolism or excretion. And (in older persons) the likelihood that they may already be managing one or more chronic medical conditions (and thus taking other medicines that could interfere with the therapy being studied). This could put patients at risk, or skew the data-driven findings related to safety, efficacy and dosing. 

Renal or hepatic function:The kidneys and liver play a crucial function in how drugs are metabolized and excreted from the human body. For this reason, study protocols often exclude patients who have impaired kidney or liver function.

Chronic health conditions: Patients with chronic illness (like heart disease or diabetes) or infectious diseases (like HIV), may have less favorable overall clinical outcomes or may experience drug to drug interactions, both of which would put the participant at risk and undermine the clinical and safety profile of the investigational therapy.

Pregnant or lactating women: For clinical and ethical reasons, many clinical trial sponsors are not willing to put pregnant women and their fetuses or nursing children at risk.

Concomitant medications: Many trials will list certain medications that participants must avoid for a defined amount of time prior to beginning and while on the trial, due to concerns of drug interactions or confounding results. If participants rely on any of these medications to manage other conditions, they would not be eligible for the trial.

Investigator’s discretion: Clinical trial investigators may be allowed to use their own judgment to exclude certain patients who they believe may be unlikely to adhere to therapy and could thus undermine the scientific integrity of the trial findings.

Striking an appropriate balance

By using inclusion and exclusion criteria, trial sponsors and investigators are able to increase the likelihood that the resulting data reflects the actual clinical and safety endpoints of the investigational therapy, and feel confident they are not exposing the participants to increased risk. However, if the criteria are too strict (thus narrowing enrollment too much), the findings will be more limited. For example, if the enrollment criteria create a bias by favoring younger patients who are inherently more healthy, then the clinical findings may not fully represent how the medication would work in patient populations who are likely to be prescribed the medication in real-world healthcare settings. Similarly, overly strict inclusion and exclusion criteria can make it hard to recruit a sufficient number of participants — especially for conditions that already have small target patient populations.

Some industry observers argue that certain eligibility criteria no longer have a clear scientific or clinical rationale, and, in some cases, may be unnecessarily restrictive. One option for overcoming the traditional limitations of strict inclusion and exclusion criteria would be to place trial participants into low-risk and high-risk groups and conduct parallel studies.

This blog is intended to be informational in nature. The information and other content provided in this blog, or in any linked materials, are not intended and should not be construed as medical advice, nor is the information a substitute for professional medical expertise or treatment.

If you have any questions or concerns, please talk to your doctor. Never disregard professional medical advice or delay in seeking it because of something you have read on this blog or in any linked materials. If you think you may have a medical emergency, call your doctor or emergency services immediately.

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