TABLE 3.   Tests for HCV infectiona Test and type Description Application(s) Comments Anti-HCV EIA and supplemental assay (i.e., recombinant immunoblot assay [RIBA]) Indicates past or present infection but does not differentiate between acute, chronic, or resolved infection; all positive EIA results should be verified by a supplemental assay Sensitivity 97%; EIA alone has low positive predictive value in low-prevalence populations HCV RNA   Qualitative testsb,c Reverse transcriptase PCR (RT-PCR) amplification of HCV RNA by in-house or commercial assays (e.g., Amplicor HCV) Detects presence of circulating HCV RNA; for monitoring patients on antiviral therapy Detects virus as early as 1-2 weeks after exposure; detection of HCV RNA during course of infection may be intermittent (a single negative RT-PCR result is not conclusive); false-positive and false-negative results might occur   Quantitative testsb,c RT-PCR amplification of HCV RNA by in-house or commercial assays (e.g., Amplicor HCV Monitor); branched-chain DNA assays (e.g., Quantiplex HCV RNA Assay) Determines concentration of HCV RNA; may be useful for assessing the likelihood of response to antiviral therapy Less sensitive than qualitative RT-PCR; should not be used to exclude the diagnosis of HCV infection or to determine treatment endpoint   Genotypingb,c Several methodologies available (e.g., hybridization, sequencing) Groups isolates of HCV based on genetic differences into six genotypes and >90 subtypes; with new therapies, length of treatment may vary based on genotype Genotype 1 (subtypes 1a and 1b) most common in United States and associated with lower response to antiviral therapy   Serotypingb EIA based on immunoreactivity to synthetic peptides (e.g., Murex HCV Serotyping 1-6 Assay) No clinical utility Cannot distinguish between subtypes; dual infections often observed a Adapted from reference 64a. b Currently not FDA approved; lack standardization. c Samples require special handling (e.g., serum must be separated within 2 to 4 h of collection and stored frozen [20 or 70°C]; samples should be shipped on dry ice).