Update pre-test probability after a positive or negative result using test rates or a known likelihood ratio for that result.
How to use this tool
- Choose the event or model and review the assumptions below.
- Enter the labeled inputs. Probability inputs are percentages from 0 to 100, not decimal fractions.
- Select Calculate and read the labeled outputs.
- Change an input and calculate again; Reset restores the example inputs.
Calculation method
Post-test odds=pre-test odds×likelihood ratio. LR+=sensitivity/(1−specificity); LR−=(1−sensitivity)/specificity.
Assumptions and limits
Educational evidence updating, not an individual diagnosis or treatment recommendation. Rates must apply to the target population and test protocol. Known-LR mode uses the likelihood ratio for the result actually observed. An impossible observed result has no posterior.
Worked example
With pre-test probability 10%, sensitivity 90% and specificity 95%, a negative result yields posterior 2/173, approximately 1.1561%. A positive result under those rates yields 2/3.
Worked reading and practical questions
- A Negative Test Updates Odds Using LR Minus
- Known Likelihood Ratio or Test Rates: Enter the Evidence You Have
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Frequently asked questions
What does this calculator assume?
Educational evidence updating, not an individual diagnosis or treatment recommendation. Rates must apply to the target population and test protocol. Known-LR mode uses the likelihood ratio for the result actually observed. An impossible observed result has no posterior.
How do I enter and update the inputs?
Use the labeled fields, then select Calculate. Percent fields use 0 to 100. After editing, calculate again; Reset restores the example inputs.