Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1384 page)

   Western blots specific for HIV-2 or HIV-1 subtypes other than M should be considered for patients at epidemiologic risk for such infections, or when HIV-1 WB gives indeterminate or negative results. HIV-2 is most commonly acquired in West Africa.

   Interpretation

   
Positive result:
As established by CDC, the interpretive criterion for HIV-1 is defined by the presence of any two of the following bands: p24, gp41, and gp120/160.

   
Indeterminate results:
HIV infection is neither confirmed nor ruled out; additional testing is required.

   Indeterminate results may be caused by factors related to HIV infection, including a weak titer of anti–HIV-1 antibodies (e.g., early seroconversion), advanced AIDS, or infection with HIV-2 or HIV-1 subtype O. Nonspecific causes include autoantibodies, hemodialysis, hypergammaglobulinemia, or recent vaccination.

   Additional testing for indeterminate results may include HIV-1 RNA testing, WB for HIV-2 or HIV-1 subtype O, or repeat HIV serology and WB in 4–6 weeks.

   
Negative result:
Negative results do not exclude the diagnosis of HIV infection; further testing is required.

   Additional testing for negative results may include HIV-1 RNA testing, WB for HIV-2 or HIV-1 subtype O, or repeat HIV serology and WB in 4–6 weeks.

   Persistent reactivity of the antibody screening assay on repeat testing with negative confirmation by WB pattern suggests nonspecific reactivity and the absence of HIV infection.

   Limitations

   HIV WB testing should be ordered only on sera that are repeatedly reactive by HIV screening EIA or rapid HIV antibody tests.

   Patients with HIV-2 infection may give indeterminate results for HIV-1 Western blots. For HIV-2 WB test, no single standard can currently be applied to all tests. The CDC recommends that each test be interpreted by the criteria suggested by the kit manufacturer.

   HIV-2 cross-reacts with HIV-1 in serologic tests. A positive screen test for HIV 1 and 2 antibodies with a repeated negative or indeterminate HIV-1 WB suggests positive HIV-2 infection and need to be confirmed HIV-2–specific test.