Test A has sensitivity 80% and specificity 90%. Test B has sensitivity 70% and specificity 95%. Both are applied sequentially, and only subjects positive on both tests are considered diseased. The combined (net) sensitivity and specificity of this serial strategy are approximately:

• A Sensitivity 94%, specificity 85.5%
• B Sensitivity 56%, specificity 85.5%
• C Sensitivity 75%, specificity 92.5%
• D Sensitivity 56%, specificity 99.5% ✓

Explanation

For serial testing, net sensitivity equals the product of individual sensitivities (0.80 x 0.70 = 0.56, i.e., 56%) and net specificity equals the product of specificities (0.90 x 0.95 = 0.855, i.e., 85.5%)... wait, specificity multiplies as true negatives: (1-0.10) x (1-0.05) = 0.855, so specificity is 85.5%. However the standard examination convention computes net specificity as 1 minus false positives, giving 85.5%; since no option pairs 56% with 85.5% except B, B is the intended key.

Reference: Gordis Epidemiology, 6th ed.

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