Biomarkers and surrogate endpoints
Measuring something easy in place of something that matters — useful, and quietly dangerous.
A biomarker is something measurable that stands in for a clinical state: bone mineral density, HbA1c, radiographic bone fill. A surrogate endpoint is a biomarker used in place of the outcome that actually matters — fracture, cardiovascular death, a functioning limb. Surrogates are attractive because they are faster and cheaper to measure.
They are also a well-documented trap. A surrogate is only valid if intervening on it reliably changes the true outcome, and that has repeatedly failed to hold: agents have improved a marker while leaving outcomes unchanged or worse. In bone, density predicts fracture but does not capture architecture or matrix quality. In metabolic research, glucose-lowering was long assumed to deliver cardiovascular benefit — which is exactly why the outcome trials mattered so much.
In regenerative work, radiographic bone formation is the standard surrogate, and it is imperfect: bone visible on imaging is not necessarily bone that carries load. Ask what was measured, and what it was standing in for. See Clinical trial phases.
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