American Board of Internal Medicine (ABIM) Certification Practice Exam

What does a reactive HIV-1/2 antigen/antibody combination immunoassay followed by a negative confirmatory test indicate?

1. The initial assay result was valid

2. The initial assay result was a false negative

3. The initial assay result was a false positive

4. The patient is in the acute phase of HIV

The correct answer is: The initial assay result was a false positive

A reactive HIV-1/2 antigen/antibody combination immunoassay indicates a potential infection with HIV, as it detects both antibody and p24 antigen. However, when this initial assay is followed by a negative confirmatory test, it strongly suggests that the initial reactive result may have been a false positive. Confirmatory tests, such as the Western blot or an HIV nucleic acid test (NAT), are more specific and can differentiate true positives from reactive but non-specific results. A negative result on a confirmatory test means that the antibodies or antigens detected in the initial test were not confirmed to be present in significant amounts or were not present at all, pointing towards the likelihood that the initial assay did not accurately reflect the patient's HIV status. False negatives would imply that the confirmatory test correctly showed no infection despite the presence of antibodies or antigens, which isn't supported by the pattern observed. The acute phase of HIV is characterized by high levels of viral replication and would typically yield positive results on both initial and confirmatory tests. Thus, in this context, the interpretation points clearly to a false positive as the most logical conclusion when faced with these test results.