USMLE Step 1 Question 359

Question:

The enzyme-linked immunosorbent assay (ELISA) for the HIV virus has a sensitivity and specificity of 99.9%. Therefore, in a population of 100,000 people where the prevalence of HIV is 1%, there will be 1 false negatives and 99 false positives. Likewise, there will be 999 true positives and 98,901 true negatives. 1000 HIV+: 999 T+ and 1 T99000 HIV-: 98901 T+ and 99 TIf an analysis was made of a blood donor pool in which blood had already been screened for HIV and now the prevelance of HIV was 0.1%, how would the sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) change?

Correct Answer: E.  Same sensitivity, Same specificity, Decreased PPV, Increased NPV

With any test, no matter its sensitivity and specificity, changing the prevalence of the disease will change the positive and negative predictive values of the test. In this case, as the prevalence decreases, positive predictive value will decrease and negative predictive value will increase.

Taking the example from the question stem, a prevalence of 0.1% would give us about 100 true positives and 100 false positives.

This is a positive predictive value of 50% as compared to 91% when the prevalence was 1%. The negative predictive value goes up a fraction of a percent, from 98901/98902 to 99800.1/99800.2.

100 HIV+: 99.9 T+ and 0.1 F99900 HIV-: 99.9 F+ and 99800.1 TSensitivity and specificity are inherent features of a test, and do not change if prevalence changes. Therefore, the answer is choice E.