Merck Manual

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Calculating Test Sensitivity and Specificity

Calculating Test Sensitivity and Specificity

Gold Standard

Disease Present

Disease Absent

Total

Test Result

Positive

a

b

a+b

Negative

c

d

c+d

Total

a+c

b+d

a = True positives; the test has correctly diagnosed the disease

b = False positives; the test is positive but disease is absent

c = False negatives; the test is negative but disease is present

d = true negatives; the test correctly diagnosed the absence of disease

Sensitivity is the true positives divided by the sum of the true positives and the false negatives: a/(a +c)

Specificity is the true negatives divided by the sum of the true negatives and the false positives: d/(b + d)