Assessment of the Cavidi ExaVir Load Assay for Monitoring Plasma Viral Load in HIV-2-Infected Patients

Abstract

HIV plasma viral load is an established marker of disease progression and of response to antiretroviral therapy, but currently there is no commercial assay validated for the quantification of viral load in HIV-2-infected individuals. We sought to make the first clinical evaluation of Cavidi ExaVir Load (version 3) in HIV-2-infected patients. Samples were collected from a total of 102 individuals living in Cape Verde, and the HIV-2 viral load was quantified by both ExaVir Load and a reference in-house real-time quantitative PCR (qPCR) used in Portugal in 91 samples. The associations between viral load and clinical prognostic variables (CD4⁺ T cell counts and antiretroviral therapy status) were similar for measurements obtained using ExaVir Load and qPCR. There was no difference between the two methods in the capacity to discriminate between nonquantifiable and quantifiable HIV-2 in the plasma. In samples with an HIV-2 viral load quantifiable by both methods (n = 27), the measurements were highly correlated (Pearson r = 0.908), but the ExaVir Load values were systematically higher relative to those determined by qPCR (median difference, 0.942 log₁₀ copies/ml). A regression model was derived that enables the conversion of ExaVir Load results to those that would have been obtained by the reference qPCR. In conclusion, ExaVir Load version 3 is a reliable commercial assay to measure viral load in HIV-2-infected patients and therefore a valuable alternative to the in-house assays in current use.

Keywords: Cape Verde; Cavidi ExaVir load; HIV-2; resource-limited settings; viral load assay.

FIG 1

Bland-Altman plot for the differences between measurements of HIV-2 viral load obtained by ExaVir Load and by qPCR. (A) The difference (ExaVir Load values minus qPCR values) is plotted against the average for each sample. The solid line represents the median, and the gray ribbon represents the corresponding 95% CI. The dotted lines represent the limits of agreement (percentile 2.5 to percentile 97.5), containing 95% of the values on the y axis. (B) The differences were regressed on averages using quantile regression. The solid line represents the regression line, and the gray ribbon represents the corresponding 95% confidence band. The regression model is annotated in the plot, as are the Wald chi-square statistic (with 1 degree of freedom) and the estimates of the regression coefficients, with the corresponding P values.

FIG 2

Comparison of the quantification of HIV-2 viral load by ExaVir Load and qPCR. The solid line represents the Deming regression line, and the gray ribbon represents the corresponding 95% confidence band. The dashed line represents the theoretical line of identity as if the measurements of ExaVir Load and qPCR were equal. The regression model is annotated in the plot, as are the estimates of the regression coefficients, with the corresponding P values.

FIG 3

Estimates for the deviation between measurements, using the model proposed for the calibration of ExaVir Load version 3 for HIV-2 viral load quantification. The y axis represents the calibration values (deviation) to be used for the adjustment of each ExaVir measurement. The solid line represents the point estimates of the deviation, and the gray ribbon represents the corresponding 95% confidence band. The horizontal dashed line represents the zero line of nonsystematic deviation. The vertical solid lines highlight the confidence bounds at the clinical decision levels described in the table below: 200 (2.3 log₁₀), 500 (2.7 log₁₀), and 1,000 (3.0 log₁₀) copies/ml.