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

- The Elecsys® HIV combi PT assay shows an improved specificity and PPV compared to its predecessor.
- The Elecsys® assay presents a similar specificity but lower PPV as the Architect® HIV Ag/Ab Combo.
- HIV tests of 4th generation are still impaired by false reactive results.
- Patient files were reviewed for underlying conditions known to be related with false positivity.
- The majority of false-positive results could be ascribed to patient-specific underlying conditions.

This study aimed at assessing the specificity of the Elecsys® 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® HIV Duo Ultra to identify false-reactive specimens prior to confirmation by quantitative PCR and line immunoassay. The Elecsys® compared to the ARCHITECT® 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 (cut-off <0.9) with the Elecsys® and with a signal to cut-off index (S/CO) between 1.09 and 12.49 (cut-off <1.0) with the ARCHITECT® were false-reactive. There was no false-reactive result with the VIDAS®. 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® or ARCHITECT® to 100% when the results of the VIDAS® 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.