In this example, a patient is assumed to have a treatment threshold (TT) of 25% for acute MI. When the probability of MI exceeds 25%, thrombolytic therapy is given. Clinicians can use the Fagan nomogram to determine when rapid echocardiography should be done before giving thrombolytic therapy. Assuming that echocardiography has a hypothetical sensitivity of 60% and a specificity of 70% for a new MI, these percentages correspond to a likelihood ratio (LR) of a positive test result (LR+) of 60/(100 − 70) = 2. A line connecting a 25% TT on the post-test probability line with LR+ (2.0) on the middle LR line intersects a pre-test probability of about 0.14. A positive test result in a patient with a pre-test probability of < 14% still results in a post-test probability of less than the TT.

Adapted from Fagan TJ. Letter: Nomogram for Bayes theorem. New England Journal of Medicine 293:257, 1975.