Test A has sensitivity 80% and specificity 90%. Test B has sensitivity 70% and specificity 95%. Both tests are applied SEQUENTIALLY, and a person is considered positive only if BOTH results are positive. The combined specificity of this two-test sequence is:

• A 85.5% ✓
• B 92.5%
• C 56.0%
• D 99.5%

Explanation

For a false positive result under sequential testing, a person without disease must be wrongly positive on both tests: 0.10 multiplied by 0.05 gives 0.005, so combined specificity is 1 minus 0.005 = 99.5%? No, combined specificity equals the probability of being correctly negative on either test, which is 1 minus the product of false positive rates: 1 - (0.10 x 0.05) = 0.995. Wait, the correct arithmetic gives 99.5%, option D. Combined specificity of serial testing is always higher than either test alone.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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