Fagan Post-test Probability Calculator (pre-test × likelihood ratio → post-test)
An evidence-based bedside reasoning tool: given a patient's pre-test probability (chance of disease) and a test's likelihood ratio, it computes the post-test probability for a positive or negative result and shows it on the classic Fagan nomogram. It helps judge "how much this test can change diagnostic certainty". Computed locally in your browser; data are not uploaded.
① Input
How to use & methodology
How do I estimate the pre-test probability?
Start from the disease's prevalence in the relevant population, then adjust up or down for the patient's symptoms, signs, and risk factors. The pre-test probability is a clinical judgment, not a fixed value; you can try a low and a high estimate to see the range of post-test probabilities.
Where do the likelihood ratios come from?
They are computed from the test's sensitivity and specificity: LR+ = sensitivity/(1−specificity), LR− = (1−sensitivity)/specificity. You can also cite LRs reported in the literature. This tool supports both input methods.
How large a likelihood ratio is useful?
As a rule of thumb, LR+ >10 or LR− <0.1 strongly helps confirm/rule out; LR+ 2–5 and LR− 0.2–0.5 are moderate; near 1 barely changes the judgment. Ultimately, see whether the post-test probability crosses your management threshold.
How does it relate to the 2×2 table and ROC tools?
The 2×2 table tool computes sensitivity/specificity/likelihood ratios from raw data; this tool applies the likelihood ratio to 'a specific patient' to obtain the post-test probability. For finding a cut-off or computing AUC on a continuous indicator, use the ROC tool.