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Introduction
A composite outcome consists of two or more component outcomes. Patients who have experienced any one of the events specified by the components are considered to have experienced the composite outcome. 1 The main advantages supporting the use of a composite outcome are that it increases statistical efficiency because of higher event rates, which reduces sample size requirement, costs, and time; it helps investigators avoid an arbitrary choice between several important outcomes that refer to the same disease process; and it is a means of assessing the effectiveness of a patient reported outcome that addresses more than one aspect of the patient's health status. 1 2 3 4 5 6
Unfortunately, composite outcomes can be misleading. This is especially true when treatment effects vary across components with very different clinical importance. 7 For example, suppose a drug leads to a large reduction in a composite outcome of "death or chest pain." This finding could mean that the drug resulted in fewer deaths and less chest pain. But it is also possible that the composite was driven entirely by a reduction in chest pain with no change, or even an increase, in death.
Studies show that treatment effects often vary, and typically, the effect is smallest for the most important component and biggest for the less important components. 3 5 8 Unless authors clearly present data for all components and take care in how they discuss composite findings, it is easy for readers to assume mistakenly that the treatment effect applies to all components. In this study, we systematically examined how composite outcomes were used and how well they were reported in recent randomised trials.
Methods
We performed a systematic review of parallel group randomised clinical trials published in 2008 that had a primary composite outcome. We excluded studies where the composite was a secondary outcome measure and studies with more than two arms.
Search strategy
An iterative search strategy was developed, using various combinations of search terms and refining them based on the initial collection of trials. Furthermore, we identified relevant terms from a previous review of cardiovascular trials published between 2000 and 2006, 8 where the authors had hand searched 14 major journals. The final PubMed search was done on 26 January 2009....