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

Abstract

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 (R²=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.

Keywords: Diagnostic; HIV-1; Nucleic acid test; Viral load.