Sensitivity and Specificity – Definitions

Sensitivity = proportion of patients WITH the diagnosis who HAVE the physical sign or test result.  (SnNOut – a Sensitive test, when Negative, rules OUT disease.)

Specificity = proportion of patients WITHOUT the diagnosis who DO NOT HAVE the physical sign or test result. (SpPIn – a Specific test, when Positive, rules IN disease.)

Image result for 2x2 table ebm

Total number of patients with disease = n1 = a+c

Total number of patients without disease = n2 = b+d

Sensitivity = a/a+c or a/n1

Specificity = d/b+d or d/n2

Positive LR [LR if finding present] = [a/n1]/[b/n2] = sensitivity / (1-specificity)

Negative LR [LR if finding absent] = [c/n2]/[d/n2] = (1-sensitivity) / specificity

Likelihood ratio = [probability of finding in patients WITH disease] / [probability of finding in patients WITHOUT disease]

Adapted from McGee (2018) Evidence-Based Physical Examination.

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