Understanding indeterminate HIV serology through antigen-band pattern analysis of INNO-LIA and recomLine HIV confirmatory assays.

Indeterminate Human Immunodeficiency Virus (HIV) serological results remain a major challenge in confirmatory diagnosis, delaying reporting and clinical decision-making. The INNO-LIA HIV I/II Score assay, a widely used confirmatory line immunoassay, frequently produces indeterminate profiles that complicate interpretation. This study evaluated the Mikrogen recomLine HIV-1/2 IgG assay against INNO-LIA, incorporating HIV-1 RNA PCR as an adjudicative reference within the Architect-reactive samples, with emphasis on antigen-band patterns underlying indeterminate results. A total of 180 archived serum samples previously tested by the Architect HIV Ag/Ab Combo and INNO-LIA were retested using recomLine; 80 Architect-reactive samples underwent HIV-1 RNA PCR testing. When only definitive results were considered, recomLine demonstrated complete agreement with INNO-LIA. When indeterminate results were included as non-concordant, specificity was 98.3% (114/116). INNO-LIA produced more indeterminate results than recomLine (13.3% vs. 2.8%), mainly due to isolated gp41 reactivity. RecomLine reclassified 85% of INNO-LIA indeterminate samples, all of tested PCR-negative. These findings provide insights into mechanisms underlying indeterminate HIV serology, suggesting that incomplete or isolated antigenic responses, particularly gp41, are major contributors. No HIV-2 monoinfection or HIV-1/2 dual infections were identified. Overall, recomLine reduced indeterminate outcomes while maintaining high diagnostic accuracy, supporting its role as a confirmatory assay.