Two neutral laboratory cards beside forward and backward arrow-shaped wooden markers. Original AI-generated artwork produced for GreenCalc; not a measured diagram or empirical data display.
Positive and negative results use different likelihoods
LR plus is sensitivity divided by one minus specificity. LR minus is one minus sensitivity divided by specificity. Each compares the probability of the observed result under the target condition with its probability without that condition. Neither ratio is itself a post-test probability.
Calculate a concrete pair
For sensitivity 80% and specificity 90%, LR+ = 0.8/0.1 = 8, while LR− = 0.2/0.9 = 2/9. A positive result multiplies pre-test odds by eight; a negative result multiplies them by about 0.222222. The two multipliers are not reciprocals.
Handle zero denominators explicitly
If specificity is one and sensitivity is positive, LR+ is infinite in the entered sample-rate model. If both numerator and denominator vanish, the ratio is undefined. These boundary sample results should not be treated as proof of perfect population behavior. The Sensitivity and Specificity Calculator preserves undefined values and does not insert an undisclosed correction.
Carry the validation assumptions into any update
Read reference standards and denominators before interpreting rates estimated from counts. The Post-Test Probability Calculator can apply an appropriate LR to an entered prior probability, but cannot establish that validation rates transfer to a particular person or setting. For repeated evidence, dependence must also be considered; multiplying unqualified marginal ratios can overstate the information.
Check the model behind your result
Use the Sensitivity and Specificity Calculator for the supported calculation and review its stated assumptions before interpreting the output. Find related tools in the Statistics category, or browse the GreenCalc homepage for other calculation topics.
By GreenCalc Editorial Team · Updated 2026-10-26