Evaluation of the performance of the Cepheid Xpert HIV-1 Viral Load Assay for quantitative and diagnostic uses

Background: Cepheid's Xpert HIV-1 Viral Load (Xpert VL), a simplified, automated, single-use quantitative assay used with the GeneXpert System, is not FDA approved. Objectives: Using stored plasma, we conducted a study to assess the ability of Xpert VL to quantify viral load relative to the Roche COBAS AmpliPrep/COBAS TaqMan HIV-1 (Cobas VL) and to examine the use of the Xpert VL as a qualitative diagnostic test. Study design: Following HIV-1 viral stock dilutions, we conducted a probit analysis to identify the concentration where 95 % of specimens had quantified VLs. We also examined Xpert and Cobas log VL correlation in linearity panels; compared the proportion of 220 seroconverter specimens with virus detected using McNemar's test; and tested specimens from persons with untreated, established HIV-1 infection (n =149) and uninfected persons (n = 497). Furthermore, we examined Xpert VL as a qualitative test in seroconverter specimens with early (n = 20) and later (n= 68) acute infections. Results: At 1.80 log10 copies/mL, 95 % of specimens had quantifiable virus using Xpert VL. Xpert and Cobas VLs were highly correlated (R-2 = 0.994). The proportion of seroconverter specimens with virus detected using Cobas and with Xpert VL was not statistically different (p = 0.0578). Xpert VL detected 97.9 % of established infections, and specificity was 99.80 % (95 % CI 98.87%-99.99%). Xpert VL detected 90 % and 98.5 % of early and later acute infections, respectively. Conclusions: If approved, Xpert VL could allow U.S. laboratories that cannot bring on large, complex testing platforms to conduct HIV monitoring. An approval for diagnostic use may provide timely identification of HIV infections.