Fagan Nomogram — Post-Test Probability Calculator
Turn a pretest probability into a post-test probability. Enter the pretest probability and the test’s positive and negative likelihood ratios to see how a positive or negative result changes the probability of disease.
Inputs
Pretest probability is often the disease prevalence or your clinical estimate. Get LR+/LR− from the diagnostic-test calculator.
Applies Bayes’ theorem (post-test odds = pretest odds × LR) — the calculation behind the Fagan nomogram. For education/reference. Computed locally.
The Bayesian idea
A test result should update, not replace, what you already believed. The nomogram converts the pretest probability to odds, multiplies by the likelihood ratio, and converts back: post-test odds = pretest odds × LR. A high LR+ makes a positive result convincing; a very low LR− makes a negative result reassuring.
Frequently asked questions
What is the Fagan nomogram?+
It is a graphical tool that applies Bayes’ theorem to convert a pretest probability into a post-test probability using a test’s likelihood ratio. This calculator does the same maths: post-test odds = pretest odds × LR.
Where do I get the likelihood ratios?+
From the test’s diagnostic accuracy: LR+ = sensitivity ÷ (1 − specificity) and LR− = (1 − sensitivity) ÷ specificity. Use our diagnostic-test calculator to compute them from a 2×2 table.
What pretest probability should I use?+
It is your estimate of disease likelihood before testing — often the local prevalence adjusted by the clinical picture. The tool then shows how a positive or negative result shifts that probability.
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