Which statement describes a scenario where a positive test may not reliably confirm the disease?

Which statement describes a scenario where a positive test may not reliably confirm the disease?

• A. Positive results may have low PPV at low prevalence.
• B. A positive result always confirms disease.
• C. A positive result indicates high NPV.
• D. A positive result means high specificity.

Answer: (A)

The key idea is that a positive test’s ability to confirm disease depends on how common the disease is in the population being tested. Positive predictive value (PPV) is the probability that someone with a positive result actually has the disease, and PPV varies with prevalence. When disease prevalence is low, most people tested don’t have the disease, so even a test with good sensitivity and specificity can produce a substantial number of false positives. This means a positive result may not reliably confirm disease in a low-prevalence setting. The other statements don’t fit because a positive result does not guarantee disease (PPV isn’t 100%), and negative-predictive value (NPV) concerns the accuracy of negative results, not positives. High specificity reduces false positives, but PPV also depends on prevalence and sensitivity; thus a positive result doesn’t automatically confirm disease even with high specificity.

The key idea is that a positive test’s ability to confirm disease depends on how common the disease is in the population being tested. Positive predictive value (PPV) is the probability that someone with a positive result actually has the disease, and PPV varies with prevalence. When disease prevalence is low, most people tested don’t have the disease, so even a test with good sensitivity and specificity can produce a substantial number of false positives. This means a positive result may not reliably confirm disease in a low-prevalence setting.

The other statements don’t fit because a positive result does not guarantee disease (PPV isn’t 100%), and negative-predictive value (NPV) concerns the accuracy of negative results, not positives. High specificity reduces false positives, but PPV also depends on prevalence and sensitivity; thus a positive result doesn’t automatically confirm disease even with high specificity.