In a clinical research trial, a clinical endpoint refers to occurrence of a disease, symptom, sign or laboratory abnormality that constitutes one of the target outcomes of the trial. The results of a clinical trial generally indicate the number of people enrolled who reached the pre-determined clinical endpoint during the study interval, compared with the overall number of people who were enrolled. Once a patient reaches the endpoint, he or she is generally excluded from further experimental intervention (the origin of the term endpoint).

For example, a clinical trial investigating the ability of a medication to prevent heart attack might use chest pain as a clinical endpoint. Any patient enrolled in the trial who develops chest pain over the course of the trial, then, would be counted as having reached that clinical endpoint. The results would ultimately reflect the fraction of patients who reached the endpoint of having developed chest pain, compared with the overall number of people enrolled.

When an experiment involves a control group, the proportion of individuals who reach the clinical endpoint after an intervention is compared with the proportion of individuals in the control group who reached the same clinical endpoint, reflecting the ability of the intervention to prevent the endpoint in question.

In clinical cancer research, common endpoints include discovery of local recurrence, discovery of regional metastasis, discovery of distant metastasis, onset of symptoms, hospitalization, increase or decrease in pain medication requirement, onset of toxicity, requirement of salvage chemotherapy, requirement of salvage surgery, requirement of salvage radiotherapy, death from any cause or death from disease. A cancer study may be powered for overall survival, usually indicating time until death from any cause, or disease specific survival, where the endpoint is death from disease or death from toxicity.

Some studies will examine the incidence of a combined endpoint, which can merge a variety of outcomes into one group. For example, the heart attack study above may report the incidence of the combined endpoint of chest pain, myocardial infarction, or death. An example of a cancer study powered for a combined endpoint is disease-free survival (DFS); trial participants experiencing either death or discovery of any recurrence would constitute the endpoint.

A surrogate endpoint (or marker) is a measure of effect of a certain treatment that may correlate with a real clinical endpoint but doesn't necessarily have a guaranteed relationship. The National Institutes of Health (USA) define surrogate endpoint as "a biomarker intended to substitute for a clinical endpoint".^[1][2]

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