A rapid screening test for chlamydial infection has a sensitivity of 90% and specificity of 80%. It is applied to a population where disease prevalence is 10%. What is the Negative Predictive Value?
- A Approximately 33.3%
- B Approximately 83.3%
- C Approximately 98.6% ✓
- D Approximately 90.0%
Correct answer: C. Approximately 98.6%
Explanation
In 1000 people, 100 have disease: 90 true positives and 10 false negatives. Of the 900 without disease, 80% (720) are true negatives. NPV equals true negatives divided by all test negatives, that is 720 divided by 730, which is about 98.6%. The tempting distractor B comes from wrongly dividing 720 by the total non-diseased population of 900, which actually gives specificity, not NPV.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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