[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Research methods](https://durgical.com/en/epidemiology/areas/research/) › Outcomes, surrogate and composite end points

# Outcomes, surrogate and composite end points

- Concept

The result a study measures to decide whether something works: hard (death), soft, surrogate (a lab value) or composite.

Updated Nov 13, 2026

## Key points

- A single primary outcome, defined in the protocol before the start.
- Surrogate: stands in for how a patient feels, functions or survives; it is trustworthy only if validated for that type of treatment.
- A surrogate can improve while the clinical outcome worsens: suppressing ectopic beats after infarction with certain antiarrhythmics raised mortality (the CAST trial).
- Composite: adds events; it increases the number of cases, but the effect may come from the least serious component.
- The components of a composite end point are always reported separately.

On this page

- The basics Key points
- What it is
- Key facts
- Worked example
- Types of outcome
- Markers
- In practice
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

The primary outcome is the end point that answers the question and on which the sample size is based. Hard outcomes (death, myocardial infarction) matter directly to patients and are measured without ambiguity; soft ones (pain, a revascularisation decided by the doctor) depend more on judgement. Surrogate end points (blood pressure, HbA1c, viral load) stand in for the clinical outcome and shorten studies, but they can mislead; composite end points add several events together to get more cases.

## Key facts

- A well-built composite: components of similar importance, with effects in the same direction and similar frequency.
- A validated surrogate: a clear biological basis plus trials showing that a change in it predicts clinical benefit.

## Worked example

- Made-up trial, 1 000 patients per group. Composite end point: death, infarction or urgent revascularisation (first event counted).
- Control: 30 deaths + 40 infarctions + 90 revascularisations = 160 events (16 %).
- Treatment: 30 + 38 + 52 = 120 events (12 %). RR of the composite = 12 / 16 = 0.75, ‘25 % fewer events’.
- By component: death 30 vs 30 (RR 1.00); infarction 38 vs 40 (RR 0.95); revascularisation 52 vs 90 (RR ≈ 0.58).
- Of the 40 events avoided, 38 are revascularisations (90 − 52) and 2 are infarctions; no deaths.
- Reading: the benefit of the composite comes almost entirely from the softest component; ‘25 % fewer events’ does not mean fewer deaths.

## Types of outcome

| Type | Example | Advantage | Risk |
| --- | --- | --- | --- |
| Hard | All-cause death | Matters to patients; unambiguous | Rare: needs large samples |
| Soft | Pain, hospital admission decided by the doctor | More frequent | Open to judgement and observer bias |
| Surrogate | Blood pressure, HbA1c, viral load | Short, small studies | May not predict real benefit |
| Composite | Death, infarction or revascularisation | More events and more power | The effect may come from the least serious component |

## Markers

- Primary
- Secondary
- Surrogate
- Composite

## In practice

- When reading a trial, ask: does the primary outcome matter to patients, or is it a laboratory number?
- Accelerated drug approvals sometimes rest on ‘reasonably likely’ surrogates and require confirmatory studies afterwards.

## Common pitfalls

- Switching the primary outcome at the end of the study to the one that came out significant.
- Presenting the effect on the composite as if it applied to each component.
- Assuming that a surrogate validated for one type of drug holds for a different one.

## Memory aid

‘Lab values do not die’: if a study measures only a test result, what happens to the patient is still unknown.

## Test yourself

Think of the answer, then tap the question to see it.

## What is a surrogate end point?

A measure (blood pressure, HbA1c, viral load) that stands in for the clinical outcome and is expected to predict it.

## Why are composite end points used?

To get more events and more power with fewer patients.

## A composite falls from 100 to 80 events, but deaths are 20 and 20. What follows?

The benefit comes from the non-fatal components; there is no evidence that mortality falls.

## How many primary outcomes should a trial have?

One, fixed in the protocol; the sample size is calculated for it.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/epidemiology/outcomes-surrogate-and-composite-end-points/)

## Connections

## Related 5

- [Choosing and defining variables](https://durgical.com/en/epidemiology/choosing-and-defining-variables/)
- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Relative risk](https://durgical.com/en/epidemiology/relative-risk/)
- [Number needed to treat](https://durgical.com/en/epidemiology/number-needed-to-treat/)
- [Statistical vs clinical significance](https://durgical.com/en/epidemiology/statistical-vs-clinical-significance/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Surrogate Endpoint Resources for Drug and Biologic Development](https://www.fda.gov/drugs/development-resources/surrogate-endpoint-resources-drug-and-biologic-development) (U.S. Food and Drug Administration, 2018); [Chapter 3: Defining the criteria for including studies and how they will be grouped for the synthesis](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-03) (Cochrane, 2026).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/epidemiology/outcomes-surrogate-and-composite-end-points/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/outcomes-surrogate-and-composite-end-points/