Test A has sensitivity 80% and specificity 90%. Test B has sensitivity 70% and specificity 95%. Both are applied in series, that is, only individuals positive on Test A undergo Test B, and both must be positive to label disease. The net sensitivity and net specificity of this sequential strategy are approximately:
- A Sensitivity 75%, specificity 92.5%
- B Sensitivity 94%, specificity 85.5%
- C Sensitivity 56%, specificity 99.5% ✓
- D Sensitivity 56%, specificity 85.5%
Correct answer: C. Sensitivity 56%, specificity 99.5%
Explanation
In series testing, net sensitivity is the product of individual sensitivities: 0.80 x 0.70 = 0.56, that is 56%. Net specificity is calculated from false positives: 1 minus (0.10 x 0.05) = 0.995, that is 99.5%. Option B gives the values for parallel testing, where sensitivities add and specificities multiply, so it kills candidates who confuse the two strategies.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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