Trial Design
Surrogate Endpoint
A surrogate endpoint is a biomarker or intermediate clinical measurement used as a substitute for a direct measure of how a patient feels, functions, or survives, that is reasonably likely to predict clinical benefit.
FDA accepts validated surrogates (HbA1c for diabetes outcomes, viral load for HIV, LDL cholesterol for cardiovascular risk) as primary endpoints in approval-track trials. Accelerated Approval permits unvalidated surrogates that are reasonably likely to predict benefit, conditioned on confirmatory post-marketing trials.
Surrogate endpoint failures (e.g., the CAST trial in arrhythmia suppression) underscore that biological plausibility alone is not sufficient; surrogate-to-outcome relationships must be established empirically.
Related terms
Primary Endpoint
A primary endpoint is the pre-specified outcome measure on which a clinical trial's conclusions about efficacy or safety are principally based, used to power the sample size calculation and to test the trial's main hypothesis.
Endpoint (Clinical Trial)
A clinical trial endpoint is a defined, measurable outcome that determines whether the intervention being studied has the intended effect; it is the variable on which the trial's success or failure is statistically evaluated.
Biomarker
A biomarker is a measurable biological characteristic (molecular, histologic, radiographic, or physiologic) used in clinical trials to indicate normal biology, disease state, treatment response, or prognosis.
Accelerated Approval
Accelerated Approval is an FDA pathway that allows earlier approval of drugs treating serious conditions and filling unmet medical need on the basis of a surrogate endpoint that is reasonably likely to predict clinical benefit, conditional on confirmatory post-marketing trials.
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