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Likelihood Ratios

Test\Disease

+

-

+

A

B

-

C

D

Sensitivity = A/A+C
Specificity = D/B+D

LR+ = sensitivity/1-specificity
LR- = 1-sensitivity/specificity

Calculate…

Pre-test probability

=>

Convert to pre-test odds

=>

Pre-test odds x LR

=

Post-test odds

=>

Covert to probability

Or use the normogram…

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