How to use
- Enter a realistic pre-test probability for the case before the new result.
- Keep both LR+ and LR− visible, then choose whether the observed result was positive or negative.
- Read post-test probability together with the odds step so others can audit how the update happened.
Wave 9 diagnostic probability
Convert pre-test probability to post-test probability
The share URL stores only the result choice and decimal places. It does not store the probability or LR values you enter.
Use LR+ only for a positive result and LR− only for a negative result. Keep both visible so the direction of update is easy to audit.
Enter pre-test probability and LR values to update the post-test probability.
Where this page fits in the diagnostic workflow
Use likelihood ratio when you still need LR+ or LR− from TP, FP, TN, and FN. Use this page after that, once the ratio is already known and you need a case-level posterior probability.
If you want to see the full probability update in Bayes theorem form, move to Bayes theorem. If you instead need predictive values at a prevalence level, use NPV & PPV.
Frequently asked questions
What does this page calculate?
It updates a pre-test probability into a post-test probability by converting probability to odds, multiplying by LR+ or LR−, and converting back into probability.
When should I use LR+ and when should I use LR−?
Use LR+ after a positive result and LR− after a negative result. The page keeps both inputs visible so you can compare how strongly each result moves the odds.
How is this different from NPV / PPV?
NPV and PPV describe predictive values at a given prevalence. This page starts from a pre-test probability and applies likelihood ratios to update one case after one observed result.
Does the share URL include my probability or LR values?
No. The share URL stores only lightweight settings such as result choice and decimal places. Input values stay in your browser.