A composite outcome in a clinical trial is an endpoint that combines two or more individual clinical outcomes into one measure of treatment effect. A participant is considered to have experienced the composite outcome if any of the predefined component outcomes occurs. Composite outcomes are commonly used in clinical trials to increase the number of outcome events, thereby improving statistical efficiency and reducing the sample size or duration required to detect a treatment effect.
In antimicrobial clinical trials, composite outcomes may combine outcomes such as all-cause mortality, clinical response and, microbiological eradication, treatment failure, need for rescue therapy, or recurrence of infection. Composite outcomes are most appropriate when the individual components are of similar clinical importance to patients, are expected to occur with comparable frequency, and are likely to be affected by the intervention in the same direction and to a similar extent. Because treatment effects may differ across the individual components, the results of each component should be reported alongside the overall composite outcome to support accurate interpretation of the trial findings.