A new rapid diagnostic test for tuberculosis is evaluated in 1000 subjects (100 with TB confirmed by culture, 900 without TB). The test is positive in 90 of the 100 TB cases and also in 90 of the 900 non-TB subjects. The negative predictive value (NPV) of this test is:
- A 98.9% ✓
- B 90%
- C 91%
- D 50%
Correct answer: A. 98.9%
Explanation
True positives = 90, false negatives = 10, true negatives = 810, false positives = 90. NPV = TN/(TN + FN) = 810/(810 + 10) = 810/820 = 98.9%. Sensitivity = TP/(TP+FN) = 90/100 = 90%; Specificity = TN/(TN+FP) = 810/900 = 90%; PPV = TP/(TP+FP) = 90/(90+90) = 50%. The high NPV (98.9%) reflects the low prevalence — a negative test reliably rules out disease.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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