Specificity of two HIV screening tests detecting simultaneously HIV-1 p24 antigen and antibodies to HIV-1 and-2

This study aimed at assessing the specificity of the Elecsys (R) HIV combi PT in comparison to the ARCHITECT. HIV Ag/Ab Combo. With both of these assays, 3997 unselected sera from patients of a tertiary health care centre in Basel, Switzerland, were screened for HIV. Reactive sera were reanalysed on the VIDAS (R) HIV Duo Ultra to identify false-reactive specimens prior to confirmation by quantitative PCR and line immunoassay. The Elecsys (R) compared to the ARCHITECT (R) shows a similar specificity (99.7% versus 99.8%) but a slightly lower positive predictive value (71.8% versus 80%). Samples tested with a cut-off index (COI) between 0.91 and 4.85 (cutoff < 0.9) with the Elecsys (R) and with a signal to cut-off index (S/CO) between 1.09 and 12.49 (cut-off < 1.0) with the ARCHITECT (R) were false reactive. There was no false-reactive result with the VIDAS (R). Of the false reactive samples, 66.7% could be related to patient-specific underlying conditions. The HIV two-tiered diagnostic algorithm proposed in this work improved the positive predictive values of the Elecsys (R) or ARCHITECT to 100% when the results of the VIDAS (R) were included. Values just above the cut-off are highly suspicious to be false-reactive and high COI or S/CO ratios are associated with true positivity.