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Prentice [1] offered a statistical denition of a surrogateendpoint as ‘a response variable for which a test of thenull hypothesis of no relationship to the treatmentgroups under comparison is also a valid test of thecorresponding null hypothesis based on the trueendpoint’. A National Institutes of Health (NIH)working group generated less quantitatively rigorousdenitions that contrasted clinical and surrogateendpoints [2]. A ‘clinical endpoint’ is ‘[a] characteristicor variable that reects how a patient feels, functions,or survives’, and so is related to the surrogate, ‘[a]biomarker that is intended to substitute for a clini-cal endpoint’. Moreover, ‘[a] surrogate endpoint isexpected to predict clinical benet (or harm or lack ofbenet or harm) based on epidemiologic, therapeutic,pathophysiologic, or other scientic evidence’ [2].While Prentice’s denition of surrogate requires thesurrogate to account for the entirety of the effect of anintervention on a clinical endpoint, the NIH denitionis more lenient. There must be clear distinction bet-ween a surrogate endpoint and a correlate. While anappropriate surrogate necessarily correlates with aclinical endpoint, all correlates are not effectivesurrogates [3]. For example, fever is a clinical correlateof serious vascular access infection, but is not anadequate surrogate for demonstrating the ability ofan intervention to attenuate access-related sepsis. So,all surrogate measures are clinical correlates, but theconverse is far from true. Surrogate endpoints areattractive for clinical trials, especially if the surrogateoutcome occurs sooner, more often, or is easier tomeasure.Themedicalliteratureisrepletewithexamplesof putative surrogates that have failed to adequatelypredict clinical endpoints [4,5]. For example, reducingventricular ectopy does not necessarily correlate withimproved mortality. In hindsight, these measures wereclinical correlates alone. This sobering limitation hasbeenobservedeveninsituationsinwhichasoundchainof logic supports the use of the measure as a surrogate.

Left ventricular hypertrophy: a surrogate end point or correlate of cardiovascular events in kidney disease?