What happens to the positive predictive value of a test if the prevalence of the disease in the tested population falls, with sensitivity and specificity unchanged?
Answer and explanation
Correct answer: D. It falls
- Step 1: Predictive values depend on prevalence.
- Step 2: Lower prevalence means more of the positive results are false positives, so PPV falls.
Answer
It falls.
Why not the others: It rises: Lower prevalence adds proportionally more false positives. It stays the same: Only sensitivity and specificity are fixed, not predictive values. It rises only if the test is repeated: Repeating the test changes the pre-test probability, not this relationship.
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