Surrogate endpointSurrogate marker · Intermediate outcome

A surrogate endpoint is a marker measured in place of a hard outcome: a blood value instead of a heart attack, a biomarker instead of a death, a NAD level instead of a healthier old age. Surrogates are used because they move faster and cheaper than real outcomes do.

It matters because a treatment can improve a surrogate and fail to improve — or actively worsen — the outcome the surrogate stands for. Medicine has learned this repeatedly and expensively. It is also the standard move in supplement marketing: demonstrate that a compound raises a number, and let the reader supply the health benefit themselves.

In practice, when a study reports a surrogate, ask whether that marker has been shown to predict the real outcome when changed by *this kind of intervention*. "Raises NAD" is a mechanism; "walks further, lives longer" is a result.

Sources

• Centre for Evidence-Based Medicine, University of Oxford: explanation of the 2011 OCEBM Levels of Evidence