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Much knowledge about the natural history of hepatitis C (HCV) infection has been gained in the last several years and treatment strategies have improved. Likewise, laboratory testing for HCV has evolved. Initially, only serologic assays were available. Screening for HCV antibody by EIA was followed by a confirmatory immunoblot, the RIBA. If both assays were positive, infection was established.
Within the past 10 years, molecular assays that directly detect viral particles have been developed. Molecular assays utilizing PCR technology can be qualitative or quantitative. Qualitative assays are more sensitive and reproducible than quantitative assays. In addition, PCR and DNA sequencing technology can now be used to establish the HCV genotype. Currently available assays and their major features are listed in the following table.
Assay
Optimal Use
HCV Antibody EIA
Initial screening test for chronic infection
HCV Immunoblot
Confirm positive antibody if PCR qualitative test negative
HCV PCR Qualitative
Most sensitive PCR assay
Detect viremia
Assess response to therapy
Detect acute infection
HCV PCR Quantitative
Establish viral load prior to therapy
HCV Genotype
Guide therapeutic decisions
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The advent of molecular assays has led to a variety of algorithms for diagnosing HCV infection and assessing therapeutic response. A recent review (NEJM 2001;345:41-52) summarized current thinking about optimal use of serologic and molecular tests. The most cost-effective algorithm is based on the following principles:
HCV infection is most often detected by HEP C ABY EIA. Positive results should be confirmed by HCV PCR QL, which detects 50 IU/mL of virus, rather than HCV PCR QT, which detects 600 IU/mL. Persons who are EIA and PCR positive are infected and viremic.
HCV RIBA should be used to confirm HEP C ABY EIA positive results if HCV PCR QL is negative. Persons who are EIA and RIBA positive but PCR negative are infected but not viremic.
HEP C ABY EIA results may be negative in HCV infected individuals who are immuno-compromised or in the acute phase of infection. HCV PCR QL will detect viremia in these individuals.
HCV PCR QT and HCV GENO should be performed prior to therapy to determine viral load and genotype, which guide therapeutic decisions. Currently, the only relevant genotype distinction is between genotype 1 and genotypes 2 and 3.
HCV PCR QL should be used at 24 weeks, 48 weeks, and 6 months after treatment to assess response.
