Fagan Post-Test Probability (likelihood ratio)
Fagan post-test probability from a pre-test probability and a likelihood ratio (or sensitivity / specificity), computed in odds space.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Input mode
- Enter a likelihood ratio
- Pre-test probability
- 20 %
- Likelihood ratio
- 10
Result: Pre-test 20% × LR 10 → post-test 71.4%.
The tool opens with these values already filled in. Replace them with your own.
What the result means
- From sens/spec
- LR+ = sens/(1−spec), LR− = (1−sens)/spec; a positive test raises and a negative test lowers the probability of disease.
What you enter
- Input mode
- Pre-test probability (0-100, exclusive)
- Likelihood ratio (likelihood-ratio mode)
- Sensitivity (sens/spec mode)
- Specificity (sens/spec mode)
How this is calculated
- Formula
- Pre-test odds = p/(1−p); post-test odds = pre-test odds × LR; post-test p = odds/(1+odds). Computed in odds space (no float clamp at p→0/1).
Fagan TJ. Letter: Nomogram for Bayes theorem. N Engl J Med. 1975;293(5):257. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.