Session 80Poster Presentations Diagnostic Tests for HIV Session Day and Time: Tuesday 1:30 - 3:30 pm Room: Hall A
663 Comparison of the New ABBOTT AxSYM HIV Ag/Ab Combo Assay with another HIV 4th Generation Assay and with Two 3rd Generation HIV Assays Under Routine Conditions in Two Independent Clinical Microbiology Laboratories W. Fierz*1, P. Erb2 1Inst for Clin Microbiology and Immunology, St Gallen, Switzerland and 2Inst for Med Microbiology, Univ of Basel, Switzerland
Background: Fourth generation (4th gen) HIV tests simultaneously detecting p24 Ag and anti-HIV-1/2 Ab are now rapidly becoming standard assays in clinical laboratories. Their major advantage over 3rd gen HIV antibody only assays is the earlier detection of primary HIV infections (PHI). The performance of a new 4th gen HIV assay, the Abbott AxSYM HIV Ag/Ab Combo (HIV Combo) was compared with another 4th gen assay the BioMérieux VIDAS HIV DUO (HIV DUO), and with the 3rd gen Roche COBAS Core Anti-HIV 1+2+O EIA (HIV 1+2+O) and Abbott AxSYM HIV 1/2 gO (HIV1/2 gO).
Methods: A total of 793 clinical samples were tested prospectively, including 20 sera from pregnant women. In addition, 70 archived CPT-plasmas from HIV-infected patients (pts) of the Swiss HIV Cohort Study and 11 seroconversion panels from PHI cases identified in our laboratories were analyzed. All samples were simultaneously tested with the 4th and 3rd gen HIV assays. Discrepant results were resolved using neutralizing p24-Ag assays, Western Blots, PCR, and by testing consecutive sera.
Results: Results obtained from both laboratories were highly comparable. Fourth generation assays correctly identified 6/793 specimens that were confirmed positive (0.76%). One PHI was newly detected that remained undetected by the 3rd gen antibody-only tests. With HIV Combo, none of the 793 sera was repeatedly false positive yielding a specificity of > 99.87%. One sample was initially false reactive. In contrast, false positivity rates were 8/793 (1%) with HIV DUO, 8/399 (2%) with HIV 1+2+O, and 0/394 with HIV1/2 gO. We found 100% diagnostic sensitivity when testing routine pt specimens, cohort plasma, and UKNEQAS quality control samples. For the early detection of 11 PHI cases HIV Combo proved to be at least as sensitive as HIV DUO. An interesting observation was found when analyzing follow-up sera of 2 pts with PHI and early HAART where HIV DUO gave transiently negative results, whereas HIV Combo consistently remained positive. In another follow-up of a PHI case HIV Combo and HIV 1/2 gO assays became transiently negative while HIV DUO remained positive.
Conclusions: The new AxSYM HIV Ag/Ab Combo assay as tested in 2 independent routine laboratories proved to be a reliable test with excellent specificity (> 99.87%) and sensitivity for early detection of primary HIV infection. During the study, a new primary infection has been detected that would have been missed by 3rd gen HIV assays.
