TimelyScholarFree Tools › Fagan Nomogram Calculator
Work out how much a positive or negative test result changes the probability of disease. Enter the pre-test probability and the test's likelihood ratios, or its sensitivity and specificity.
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| Post-test probability if positive | 52.9%LR+ = 4.50 |
|---|---|
| Post-test probability if negative | 3.0%LR− = 0.125 |
| Pre-test odds | 0.250Post-test odds = pre-test odds × LR |
Prevalence or clinical judgement.
Sensitivity and specificity, or likelihood ratios.
And download the nomogram.
Fagan's nomogram applies Bayes' theorem graphically: a straight line from the pre-test probability through the likelihood ratio gives the post-test probability.
Likelihood ratios well above 1 raise the probability of disease after a positive result, and ratios well below 1 lower it after a negative result. The same test is much more informative in some settings than others because the pre-test probability matters.
The further from 1, the more a result changes the probability. How much change is useful depends on the pre-test probability and the clinical decision.
Yes. Enter the pooled sensitivity and specificity, or pooled likelihood ratios, to show what the test means in practice.