Interpreting composite outcomes in trials

The potential problems with the use of composite outcomes in clinical trials are well known.1 The linked systematic review by Cordoba and colleagues (doi:10.1136/bmj.c3920) indicates that many of these challenges are long lasting.2 This review of 40 trials published in 2008 that reported a composite primary outcome found little justification for the choice of outcome, imbalance in the importance of different components in 70% of trials, and problems in the definition and reporting of composite outcomes in 40% of trials. Before we abandon composite outcomes because of the authors’ assertion that their use leads to much confusion and bias,2 we should consider carefully the opportunity cost and tackle some commonly held misconceptions on composites.

A key advantage of using composite outcomes is that they can increase statistical efficiency and enable us to answer questions that could not otherwise be tackled. Increased efficiency means that trials may be smaller or of shorter duration, allowing effective treatments to be available to patients in a timely manner.1

Cordoba and colleagues also question the rationale for the choice of components of the composite.2 Consistency in the use of composites for a particular disease undoubtedly aids interpretation. Thus, in the development of new treatments in heart failure and subsequently in cardiovascular disease more generally, the composite outcome of unplanned …