Fig. 2 (A) Scatterplot of log10 HCV RNA IU/mL values as determined by VERSANT HCV RNA 3.0 and Cobas Amplicor HCV Monitor 2.0 assays in 165 clinical samples. In the insert, the conversion equation and the correlation coefficient calculated by nonparametric regression analysis are given. (B) Classification of test results obtained by both methods in sera of 118 patients chronically infected with HCV genotypes 1 and 4 according to the decision threshold of log10 5.9 IU/mL (i.e., 800,000 IU/mL). Numbers of discrepant results are indicated by underscore.
In contrast to the situation in HBV DNA viral load determinations, commercially available packages for HCV RNA quantitation have already been universally calibrated against an international WHO standard. These attempts, however, did not completely lead to the expected widespread comparability of the results obtained with different methodologies. When testing 165 sera from patients infected with HCV in parallel with both HCV RNA bDNA 3.0 and the PCR-based Cobas Amplicor HCV Monitor™ 2.0 assay (Roche Diagnostics, Branchburg, NJ), we recorded a mean absolute difference between the results of log10 0.23 (i.e., 1.7-fold) (Fig. 2A). Eleven percent of the paired results differed by even more than threefold, i.e., a deviation that is generally regarded as clinically important in consecutive viral load determi-nations. Such discrepancies may be particularly problematic in the clinical context if absolute HCV RNA concentrations are used for decision making. For instance, until recently a threshold value of 800,000 IU HCV RNA/ mL was applied to tailor the initial duration of antiviral combination therapy in naive patients chronically infected with HCV genotypes 1, 4, and 5. Agreement between the HCV bDNA 3.0 and HCV Monitor assays in predicting viral loads either above or below 800,000 IU HCV RNA/mL (i.e., log10 5.9) according to our results was only moderate, because about 14% of the samples were classified differently by both tests (Fig. 2B) and these misclassifications would have led to a wrong assignment of the initial duration of antiviral treatment in a considerable number of patients. Germer and coworkers reported very similar observations,1-15-1 indicating that HCV RNA concentrations close to any clinically defined threshold values should generally be interpreted with caution.
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