The process of diagnosis is complex and poses many challenges to doctors and other clinicians attempting to practise in an evidence based manner. When making a diagnosis in patients who are already ill we should be able to draw on evidence about the accuracy of diagnostic tests. When trying to make an early diagnosis of presymptomatic disease in well people through population screening we rely on evidence from randomised trials on whether patients benefit from such screening. Looking for presymptomatic disease among patients with an unrelated disorder (case finding) or trying to generate a differential diagnosis for patients' signs and symptoms require different types of evidence. We need to match each question to the type of evidence by using a diagnostic strategy.

Furthermore, diagnosis seldom relies on a single test. Ideally a clinician would like to find valid evidence about a cluster of tests, including the clinical examination, along with measures …