Surrogate, Clinical and Composite Endpoints: Validity, Interpretation and Reporting

A clinical endpoint directly measures how a patient feels, functions or survives. A surrogate endpoint is a biomarker or intermediate measure substituted for a clinical endpoint on the assumption that changing the surrogate will change clinical outcomes. A composite endpoint combines several individual endpoints into one measure, typically counting a participant as an event if any component occurs.

Validation of surrogate endpoints beyond correlation

Fleming and DeMets set out a now-classic warning: a surrogate endpoint is a valid substitute for a clinical endpoint only if intervening on it reliably predicts the effect on the clinical outcome, which requires that the surrogate lie on the full causal pathway between the intervention and the outcome, not merely that the two are statistically correlated in observational data1. Their review documents several instances in which treatments improved a surrogate yet failed to improve, or worsened, the actual clinical endpoint, including cases in the treatment of cardiac arrhythmia.

Defining characteristics of a clinical endpoint

A clinical endpoint directly measures an outcome of importance to the patient without requiring any further assumption about its relationship to patient wellbeing, such as survival, symptomatic relief, functional status, or a major clinical event such as stroke or myocardial infarction. Surrogate endpoints are used, particularly in early-phase or time-limited trials, because they can be measured sooner and in smaller samples; however, Fleming and DeMets argue that the trade-off (gains in speed and sample size against uncertainty about whether the surrogate truly predicts clinical benefit) should be stated explicitly rather than glossed over1.

Reporting the components of composite endpoints

Freemantle and colleagues note that composite endpoints are used to increase statistical power by combining several individual events into a single, more frequent outcome, but that this entails an interpretive cost: the composite result can be driven by its most common component, which may be the least clinically severe, while a rarer but more serious component shows no effect or even an unfavourable trend2. Reporting guidance accordingly expects the individual components of any composite endpoint to be reported alongside the combined result.

Key points

Practical application

Frequently asked questions

@misc{surrogate_clinical_composite_endpoints_2026,
  author       = {{Pubrica Academy}},
  title        = {Surrogate, Clinical and Composite Endpoints: Validity, Interpretation and Reporting},
  howpublished = {Pubrica},
  year         = {2026},
  month        = {oct},
  url          = {https://pubrica.com/academy/concepts-definitions/surrogate-clinical-composite-endpoints/},
  note         = {Accessed: <span data-cite-acc="iso">2026-10-15</span>}
}

[ ] Endpoint type (clinical/surrogate/composite) stated explicitly
[ ] Evidence for surrogate validity cited, if a surrogate is used
[ ] Individual components of any composite endpoint reported alongside the combined result
[ ] Discordance between composite components discussed explicitly