A surrogate marker (or surrogate endpoint) is a biological or clinical measurement used as a substitute for a direct measure of how a patient feels, functions, or survives. It’s often used in clinical trials to predict the effect of a treatment without having to wait for the actual clinical outcome (like survival or disease progression).

Example: Blood pressure is a surrogate marker for the risk of heart attack or stroke. HbA1c (glycated hemoglobin) is a surrogate marker for long-term glucose control in diabetes. Why use surrogate markers? They can be measured more quickly, easily, or cheaply than true clinical outcomes. They help assess drug efficacy or intervention success early in a study. However, not all surrogate markers are validated — meaning not all of them reliably predict actual clinical benefit.