Surrogate Endpoints vs Real Outcomes: When the Marker Moves but You Don't Get Better

Summary

A **surrogate endpoint** is a lab marker (LDL, HbA1c, bone density, blood pressure, tumour size) used as a stand-in for the thing you actually care about (heart attack, fracture, living longer, living better) — and the recurring, sometimes lethal lesson of medicine is that **the marker can move in the "right" direction while patients do worse**. Surrogates are genuinely useful and a few are validated, but a moved marker is a *hypothesis*, not a proven benefit, until it's tested against hard outcomes.