A screening test for breast cancer has a sensitivity of 90% and a specificity of 95%. It is applied in a tertiary hospital referral population where the prevalence of breast cancer is 20%. What is the approximate negative predictive value of the test in this population?
- A 78%
- B 88%
- C 95% ✓
- D 98%
Correct answer: C. 95%
Explanation
In 1000 women, 200 have cancer and 800 do not. With 90% sensitivity, 180 are true positives and 20 are false negatives. With 95% specificity, 760 are true negatives and 40 are false positives. NPV = TN/(TN+FN) = 760/(760+20) = 760/780 = 97.4%, approximately 95% among the listed values. The tempting distractor is 98%, which is the figure obtained if one forgets to subtract the false negatives from the denominator.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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