Patients and society expect physicians to base their approach to any type of clinical problem on informed diagnostic reasoning. Informed diagnostics means that clinicians understand and readily apply the principles of diagnostic decision making, which include an estimate of the pre-test probability (prevalence) of diseases and information about the characteristics and discriminatory power of the applied investigations. Despite the crucial importance of the appropriate use of diagnostic tools in clinical decision-making, many diagnostic tests have not been subjected to rigorous evaluation to establish the diagnostic accuracy and clinical utility of laboratory investigations using modern standards of clinical epidemiology. The lack of good quality research in the field not only contributes to inappropriate utilization of laboratory services but also to wasting significant resources. Evidence-based laboratory medicine tries to combat this problem by combining methods from epidemiology, biostatistics, clinical and social sciences with basic sciences to evaluate the role of investigations in clinical decision making and outcomes for patients. Evidence-based laboratory medicine aims to advance clinical diagnosis and management of diseases through systematic researching and disseminating generalisable new knowledge which meets the standards of critical review on clinically effective practice of laboratory investigations. The main phases of practicing evidence-based laboratory medicine are 1. question formulation, 2. systematic literature search, 3. critical appraisal of literature, 4. implementation of evidence into practice, and 5. evaluation of impact in a clinical audit cycle. The use of evidence in laboratory medicine requires systematically compiled databases of standardised and critically appraised information on the test characteristics and diagnostic accuracy of laboratory investigations. Such an approach is the Bayes Library that provides information on the global and specific measures of test performance in different patient groups and settings, in addition to prevalence data of common diseases. The Bayes Library will support informed diagnostic decisions and improve patient outcome by integrating evidence-based medicine into the diagnostic service, education and training of laboratories. The process of constant questioning and reviewing the evidence for rational diagnosis of diseases provides a practical tool to identify gaps in our knowledge and thus it generates new research ideas in laboratory medicine.