A two-stage screening protocol uses Test 1 with specificity 90%, followed by Test 2 with specificity 80% applied only to those positive on Test 1. What is the combined (net) specificity of this sequential strategy?
- A 72% ✓
- B 85%
- C 98%
- D 170%
Correct answer: A. 72%
Explanation
In serial (sequential) testing, a person must be negative on either test to be called negative overall, so the net specificity equals the product of individual specificities: 0.90 multiplied by 0.80 equals 0.72, or 72%. Serial testing therefore gains specificity at the expense of sensitivity, which is why it suits confirmatory pathways. Option D is arithmetically meaningless and option C would require multiplying sensitivities instead.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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