A community health survey finds that a new ELISA test for leprosy has sensitivity 78% and specificity 92%. When used to screen a population with a disease prevalence of 2%, what phenomenon MOST challenges the utility of this test?

• A High sensitivity will produce too many false negatives in low-prevalence settings
• B Specificity of 92% is insufficient for any screening purpose
• C ELISA cannot be used for community screening
• D Low disease prevalence dramatically reduces PPV, generating many false positives per true positive ✓

Explanation

In a population of 10,000 with 2% prevalence: diseased=200, healthy=9800. TP=0.78×200=156; FP=0.08×9800=784. PPV=156/(156+784)=156/940=16.6%. Despite reasonable sensitivity and specificity, low prevalence causes the number of false positives (784) to vastly exceed true positives (156), making confirmatory testing essential and single-screen programmes operationally demanding. This is the base-rate fallacy in diagnostic testing.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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