
%0 Journal Article
%A Arroll, Bruce 
%A Allan, GM 
%A Raina Elley, C 
%A Kenealy, Tim 
%A McCormack, James 
%A Hudson, Ben 
%A Hoare, Karen 
%T Viewpoint: Diagnosis in primary care: probabilistic reasoning
%J Journal of Primary Health Care
%D 2012/06/01
%V 4
%N 2
%P 166-173
%K 
%R https://doi.org/10.1071/HC12166
%U https://www.publish.csiro.au/paper/HC12166 
%X This article develops the concept of probabilistic reasoning as one of the techniques clinicians use in making a diagnosis. We develop the concept that every question and every examination is a diagnostic test ultimately leading to a rule in or rule out of a diagnosis. We also develop the concept of pre-test probability pointing out that false positive tests are an issue in low-prevalence settings and false negative tests are a problem. Investigative tests work best in medium-prevalence settings. The purpose of taking a history and conducting an examination is to increase the pre-test probability to a point where either treatment is commenced or more expensive/time-consuming/dangerous tests are indicated. Pre-test probabilities on their own can be used to rule out conditions. We also show how pre-test probabilities relate to the Fagan nomogram which enables visualisation of large changes in post-test probabilities which can lead to treatment/further investigation. KEYWORDS: Likelihood ratio; pre-test and post-test probability; diagnostic accuracy; probabilistic reasoning 
