A new rapid serological test for syphilis is evaluated in 1000 pregnant women in whom 100 actually have the disease. The test is positive in 85 of the 100 infected women and in 180 of the 900 uninfected women. The sensitivity and specificity of the test respectively are:
- A 85% and 80% ✓
- B 80% and 85%
- C 32% and 97%
- D 47% and 95%
Correct answer: A. 85% and 80%
Explanation
Sensitivity is the proportion of diseased persons detected: 85 of 100 equals 85%. Specificity is the proportion of non-diseased persons correctly identified: 720 of 900 equals 80%. Option C gives the predictive values: PPV is 85 divided by 265, about 32%, and NPV is 720 divided by 735, about 98%. Option D uses the total positives and negatives as denominators, a common error. Always take denominators from the true disease status columns.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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