Background:  In the United States, confirmatory testing for HIV relies on either HIV Western blot or immunofluorescence assays. Newer HIV screening tests, including rapid tests, can be more sensitive than these confirmatory tests. Assays that detect both p24 antigen and antibodies (including immunoglobulins IgM and IgG) are the most sensitive serologic tests during early infection. We evaluated strategies to confirm an antigen/antibody combination test using current FDA-approved screening tests.

Methods:  We blinded and tested 7333 specimens by Abbott Diagnostics using the ARCHITECT^® HIV Ag/Ab Combo assay (Ag/Ab; not available for sale in United States); these results were compared to 2 IgG- and IgM-sensitive, antibody-only, immnoassays (one, IA1, from Abbott, and one, IA2, from a different manufacturer), Western blot, and 6 rapid tests, to assess the ability of each to correctly resolve Ag/Ab-reactive results as the second test in a 2-test serologic algorithm. Not all specimens had results for every test. The standard for HIV⁺ specimens was a Western blot-positive result or HIV detection by the Genprobe APTIMA HIV-1 RNA test. Proportions of Ag/Ab-reactive specimens correctly resolved by each test were compared using McNemar’s test.

Results:  As many as 1378 HIV⁺ specimens reactive by the Ag/Ab test exhibited results on other tests. Between 97.1% (776 of 799) and 99.2% (1266 of 1276) of these were correctly resolved as positive by the other serologic test used. The Ag/Ab test was reactive on 14 specimens with detectable HIV-1 RNA that were not Western blot-positive (10 Western blot-negative, 4 Western blot-indeterminate); 9 and 6 were reactive on IA1 and IA2, respectively; and 0 to 4 were reactive on rapid tests. The Ag/Ab test was initially reactive on 42 specimens that were negative by APTIMA and Western blot, all of which were correctly identified by each of the rapid tests as HIV^–. When tested with IA1, 15 specimens (36%) remained concordantly false-reactive. Using IA2, 1 (2.3%) was initially reactive but was negative when repeated in duplicate. Concordance with the HIV⁺ standard was higher (p = 0.02) for IA2 than Western blot.

Conclusions:  In this study, FDA-approved HIV screening tests, including rapid tests, used in a 2-test serologic algorithm effectively confirmed 97 to 99% of the reactive Ag/Ab Combo assay results, performing as well or better than an HIV-1 Western blot in this role. Testing for HIV RNA or follow-up may be necessary to resolve the small number of early infections detected by this Ag/Ab Combo assay that are missed by less sensitive serologic tests.