different outcomes (all other participants in the study would have the same test result, irrespective of which test they were allocated to). Statistical power therefore depends on only the numbers of participants in A and B (particularly A); for example, if disease prevalence was 20%, and test 2 improved sensitivity by 20%, only 4% of the total sample size would fall in A 34 Conclusion Establishing benefit to patient health must be the priority for diagnostic evaluations. Test accuracy is one component of test evaluation, but does not capture the impact of tests on patients. By considering the ways in which tests affect patients' health, we reiterate the complex intervention perspective 30 that it is not sufficient to measure outcomes, but rather it is essential to understand how these outputs are created, by conducting analyses of their workings and the mechanisms that underpin them. Clearly, this process must be undertaken with expert and stakeholder consultation to ensure all influential mechanisms are identified. Summary points The value of diagnostic tests ultimately lies in their effect on patient outcomes Tests can affect patient health by changing diagnostic and treatment decisions, affecting time to treatment, modifying patient perceptions and behaviour, or putting patients at risk of direct harm Improved accuracy is not always a necessary prerequisite for improving patient health, nor does it guarantee other downstream improvements All elements of the management process (including decision making and treatment) must be considered when evaluating a diagnostic test Randomised controlled trials of tests can measure