Avidity Index for anti-HIV antibodies: comparison between third- and fourth-generation automated immunoassays

The development of assays for detecting recent HIV infections has become crucial for analyzing trends in infection in different populations, both for surveillance and prevention activities. The anti-HIV avidity index (AI), measured with third-generation immunoassays (which detect anti-HIV antibody), has been shown to be an accurate tool for discriminating recent HIV infections (<6 months) from established infections (≥ 6 months). We compared a third-generation immunoassay (AxSYM HIV 1/2 gO; Abbott Diagnostics) to a fourth-generation immunoassay (Architect HIV Ag/Ab Combo; Abbott Diagnostics; which detects anti-HIV antibody and p24 antigen) in terms of AI performance in distinguishing between recent and established HIV infections. A total of 142 samples from 75 HIV-infected individuals with an estimated date of seroconversion were assayed. The two assays showed the same accuracy in identifying a recent infection (91.5%), using an AI cutoff of 0.80, although Architect HIV Ag/Ab Combo was slightly more sensitive (89.4% versus 84.8%; P > 0.05) and yet less specific (93.4% versus 97.4%; P > 0.05). The correlation between assays was high (r = 0.87). When 20 specimens falling in the gray zone around the cutoff point (0.75 ≤ AI ≤ 0.84) were excluded, the accuracy of AI with Architect HIV Ag/Ab Combo was 94.7%, and the concordance between the two assays was 99.2%. The anti-HIV AI is a serological marker that accurately discriminates recent from established HIV infections. It can be successfully applied on fully automated fourth-generation HIV Ab/Ag immunoassays, which have several advantages, including increased throughput, high reproducibility, no need for specific technical skills, and easy comparability of results obtained in different settings.