Impact of HIV-1 RNA Assay Selection on Low-Level Viremia and Clinical Trial Endpoint Interpretations

Background: Accurate plasma HIV-1 viral load quantification is essential for monitoring responses to antiretroviral therapy (ART) and defining efficacy in clinical trials. Differences in assay performance characteristics may result in discordant results and affect clinical interpretation. We evaluated concordance between the Roche cobas 6800 and Abbott RealTime HIV-1 RNA assays in 2 ongoing, multicenter, phase 3b trials. Methods: Stored paired plasma aliquots from adults with HIV-1 who were virologically suppressed or initiating ART (271 and 348 paired samples, respectively) were batch re-tested using the RealTime assay; sample selection included all specimens from pre-specified key study visits and was enriched for selected centrally reported cobas 6800 results ≥50 copies/mL. Concordance was evaluated at clinically relevant categorical thresholds (50 and 200 copies/mL). Results: Overall, among samples with cobas 6800 HIV-1 RNA results ≥50 copies/mL, 92% (91/99) of samples from participants who were virologically suppressed and 68% (159/233) from participants initiating ART were <50 copies/mL by RealTime. Among samples with cobas 6800 HIV-1 RNA results ≥200 copies/mL, 78% (14/18) and 83% (44/53), respectively, were <200 copies/mL by RealTime. Most samples classified as <50 copies/mL by cobas 6800 were <50 copies/mL by RealTime. All samples classified as <200 copies/mL by cobas 6800 were <200 copies/mL by RealTime. In both studies, per-protocol case review identified no clinical contributors in most cases of viremia. Conclusions: Marked discordance between assays was observed near clinically relevant low HIV-1 RNA thresholds. These findings highlight the importance of assay-specific interpretation of low-level viremia in trials and routine care, particularly near decision-making thresholds.