The serologically negative window period at the time of HIV seroconversion may be reduced by HIV p24 antigen or viral nucleic acid assays. Recently combined antigen/antibody assays have been developed in the enzyme-linked immunosorbent assay format .
A 32-year-old man was admitted with febrile pharyngitis. He had been in a stable homosexual relationship for 9 years, and was subjected to piercing during a trip to Greece. As shown in Table 1, the routine detection of anti-HIV-1 and anti-HIV-2 antibodies with Axsym HIV 1/2 g O (Abbott, Illinois, USA) was negative on the first specimen. The tests performed 3 days later with two combined antigen/antibody assays are also shown in Table 1 : the Genscreen plus HIV antigen/antibody (Ag/Ab) (Bio-Rad, Marnes la Coquette, France) was positive with low reactivity, whereas the Vironostika Uni-form HIV Ag/Ab (Organon Teknika, Fresnes, France) was negative. The sample was not confirmed positive by Western blotting. A primary HIV infection was confirmed by testing the HIV-1-RNA viral load (HIV-1 Amplicor Monitor 1.5, Roche, Branchburg, NJ, USA) at 678 000 copies/ml. HIV-1 proviral polymerase chain reaction was also positive in the gag, pol and env genes.
We tested the consecutive samples from this HIV-1 seroconversion case with three HIV antigen/antibodies combined screening assays and one antibody assay. The three combined assays were Genscreen plus HIV Ag/Ab (Bio-Rad), and Vironostika Uni-form HIV Ag/Ab (Organon Teknika) used for routine HIV diagnostic screening, and the Vidas HIV Duo (BioMérieux, Lyon, France). The antibody assay was the Axsym HIV 1/2 gO assay (Abbott). HIV-1 p24 antigen was also assayed using the Genetic Systems HIV-1 antigen enzyme immunoassay (Bio-Rad).
We compared the HIV-1 p24 antigen limit of detection of the three combined assays and the single antigen assay with three dilutions of HIV-1 p24 antigen National Institute for Biological Standards and Control (NIBSC) reference (1000 arbitrary units; AU) (Table 1).
The HIV-1 p24 antigen/anti-HIV antibodies combined assays theoretically offer an earlier diagnosis of HIV infection than assays based only on the detection of specific antibodies, even if the yield in terms of a reduction of the window period may vary. Surprisingly in this seroconversion case, the Axsym antibodies assay was positive 3 days after the first sample, whereas the Vironostika combined assay remained negative. The other two combined assays were already reactive with the first sample.
Looking at the results with the HIV-1 p24 antigen NIBSC reference dilutions (Table 1), the Genetic Systems immunoassay remained positive with all dilutions. The Vidas Duo assay seems to be the more sensitive combined assay for antigen detection because it remained positive with HIV-1 p24 antigen diluted 200 times. The Vironostika Ag/Ab assay and the Genscreen Ag/Ab assay did not detect the HIV-1 p24 antigen diluted 200 times. The observed HIV-1 p24 antigen sensitivity is approximately 100 or 30 pg/ml for Vironostika Uni-form HIV Ag/Ab with or without 1 h of agitation, respectively , 140 pg/ml for Genscreen plus HIV Ag/Ab, 18 pg/ml for Vidas HIV Duo and 8 pg/ml for Genetic Systems HIV-1 antigen enzyme immunoassay (manufacturer's package inserts).
A second similar case was recently observed in our laboratory: we received a serum sample, positive using the Axsym HIV 1/2 g O and Vidas HIV Duo. In our hands the Vidas HIV Duo remained positive but the Vironostika Uni-form HIV Ag/Ab was negative on the same specimen. A diagnosis of primary HIV infection was confirmed by a high viral load.
Combined assays appear less sensitive than HIV p24 antigen tests for the detection of this marker, and our results indicate that they may also be less sensitive than single assays for HIV antibodies, particularly in seroconversion cases. Combining the detection of both markers seems to lead to less sensitive assays than single antibody or antigen assays .
Combined assays are theoretically capable of reducing the diagnostic window at the start of infection, but their differences in sensitivity necessitate individual evaluation of these tests for both their antigen and their antibody component.
Benoît Kabamba Mukadia
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