a biomarker or intermediate outcome used in place of a clinically meaningful final outcome (such as survival, symptom relief, or improved function). It is chosen because it is believed, based on scientific evidence (epidemiologic, therapeutic, pathophysiologic, etc.), to predict the effect of an intervention on the true clinical outcome of interest.
Example
In a cardiovascular drug trial, blood pressure reduction may be used as a surrogate endpoint for the risk of heart attacks and strokes. Lowering blood pressure predicts a lower incidence of these events, even before actual event data are available.