Evidence Based Emergency Medicine; Part 4: Pre-test and Post-test Probabilities and Fagan’s nomogram

E mergency physicians face numerous questions regarding proper management of patients and selection ofthe best laboratory test or imaging every day. Knowledge on screening performance characteristics of the diagnostic tools used in this department plays an important role in finding the answers to these questions. We assessed these characteristics in the previous parts of educational article series in Emergency journal (1-3). In the present manuscript, we will describe how to use these screening characteristics for selecting the right diagnostic tools. Overall, in managing a patient in emergency department (ED), commonly the most probable cause of the problem is considered and diagnostic and treatment measures are based on that. There are various sources for determining the initial probability of a cause, called pre-test probability in this article. Usually, the examiner’s experience, prevalence of the disease at the time of patient presentation, and clinical decision rules are the 3 most important and major sources in determining pre-test probability. Pre-test probability is either high enough to guarantee the initiation of treatment, or low enough to disprove the presence of the disease. A problem arises when this probability is average and one or more diagnostic tools are required for confirmation. Therefore, our aim when selecting a suitable test, is practically converting anaverage pre-test probability to a higher post-test probability that guarantees initiation of treatment or intervention. The most important characteristic among screening performance characteristics of a test used for this purpose is likelihood ratio. Theoreti-