Genotype dependence of hepatitis C virus load measurement in commercially available quantitative assays

Standardization and genotype independence of methods used to quantify hepat itis C virus (HCV) RNA in clinical specimens are necessary for accurate ass essment of the role of HCV quantitation as a prognostic marker for HCV infe ction and monitoring of the response to antiviral treatment. Commercially a vailable methods used to measure HCV loads include PCR-based (Roche Monitor ) and hybridization-based (Quantiplex bDNA-2) methods. Recently, a new vers ion of the Roche Monitor assay (version 2.0) has become available; it has b een modified to achieve more equal quantitation of different HCV genotypes, Consistent,vith previous reports, Roche Monitor version 1.0 substantially underestimated concentrations of RNA transcripts of types 26, 3a, 4a, 5a, a nd 6a and virus loads in individuals infected with genotypes 2 to 6 relativ e to reference tests. However, version 2.0 achieved equivalent quantitation of each genotype over a narrow quantitative range (10(3) to 5 x 10(5) copi es of RNA/ml) but significantly underestimated RNA concentrations above thi s range. The assay showed an equivalent inability to quantify high levels o f HCV RNA in plasma samples, and this was responsible for the falsely narro w range of virus loads detected in HCV-infected individuals. In contrast, t he Chiron bDNA-2 assay could only measure RNA concentrations in the upper q uantitative range (2 x 10(5) to 5 x 10(7) copies of RNA/ml) but showed equi valent sensitivity for genotypes 1 to 5; however, concentrations of type 6a RNA transcripts and virus loads in clinical specimens from individuals inf ected with type 6a were underestimated by a factor of 2 to 4, Differences w ere observed between PCR- and hybridization-based assays in their relative quantitation of HCV RNA transcripts and HCV genomic RNA, which may cause pr oblems with the use of transcripts for interassay calibration.