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Overdiagnosis — USMLE Step 3 MCQ

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HardPreventive Medicine, Epidemiology, BiostatisticsOverdiagnosisUSMLE Step 3

Two independent screening tests are used in parallel, so either positive result makes the combined strategy positive. Test A has sensitivity 90% and specificity 90%; test B has sensitivity 80% and specificity 95%. What are the combined sensitivity and specificity?

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Correct answer: C — Sensitivity 98%; specificity 85.5%

The best answer is “Sensitivity 98%; specificity 85.5%”. For parallel independent tests, a diseased patient is missed only if both tests are false negative: 1 − (0.10 × 0.20) = 0.98. A nondiseased patient is negative only if both tests are true negative: 0.90 × 0.95 = 0.855. Thus parallel testing raises sensitivity while reducing specificity.

Reference: CDC Principles of Epidemiology in Public Health Practice: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/SS1978.pdf