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Hepatitis C virus (HCV) is the most frequent cause of parenterally transmitted and sporadic chronic non-A, non-B hepatitis. As many as 175,000 people acquire hepatitis C annually in the U.S. The most commonly identified source of exposure is intravenous drug abuse, accounting for 40% of cases. Blood transfusion is responsible for only 5% of cases. The majority of cases have no identifiable exposure source. Unlike hepatitis B, sexual and maternal-fetal transmissions are uncommon. Acute hepatitis C is generally a benign disease. In transfusion transmitted infections, where the acute onset is best documented, 70% of cases are anicteric and asymptomatic; 30% have a bilirubin greater than 2.5 mg/dL and the mean peak ALT is 700 U/L. However, patients with community acquired acute HCV, usually present with overt clinical illness; 70% are icteric and 75% have ALT levels that exceed 15 times the upper limit of normal. Approximately 50% of infected individuals evolve into chronic liver disease. Chronic hepatitis C infection is the primary indication for liver transplantation. Laboratory diagnosis of HCV infection depends upon detection of circulating antibodies. The first enzyme immunoassay (EIA), which detected antibody to a single viral antigen, was introduced in 1989. In 1992, this screening test was replaced by a more sensitive second generation EIA, which detects antibody to several viral antigens. In addition, a recombinant immunoblot assay (RIBA) was licensed as a supplemental test to confirm EIA positive results. Many problems exist with current HCV antibody tests. False positive EIA results occur due to nonspecific reactions associated with storage of sera, hypergammaglobulinemia, paraproteinemia, rheumatoid factor positive sera and other autoimmune diseases, and recent influenza vaccination. Only 30-40% of EIA reactive samples are confirmed by RIBA in a low risk population. Among high-risk populations, or in patients who are both EIA reactive and have elevated ALT levels, the confirmation rate increases to 70%. Unfortunately, a significant number of EIA positive samples are not adequately resolved by RIBA and are reported as indeterminate. Another problem with antibody testing is its insensitivity. A long lag time exists between infection and antibody detection. Only 40% of patients have detectable antibody within 10 weeks of infection and 80% have demonstrable antibodies at 15 weeks. The window period may be as long as 24 weeks. Transplant patients and immunosuppressed patients may not produce detectable antibody following infection. Another striking feature of HCV is its genetic heterogeneity, which has produced multiple mutant viruses. Patients infected with less common variants mount different antibody responses that may not be detected by current EIA and RIBA, since these tests are based on the predominant HCV genotype found in the U.S. Finally, HCV antibodies persist for life in chronically infected patients and rarely disappear on recovery. Therefore, HCV antibody tests cannot distinguish between current, resolving, or past infections. Many of these problems can be resolved with subsequent testing by polymerase chain reaction (PCR) for hepatitis C RNA. PCR testing has been available since September 1995. In this assay, a DNA copy of viral RNA is synthesized by reverse transcription. This DNA molecule is amplified millions of times by PCR. The sensitivity of PCR is approximately 1000 viral genomes per mL of blood. Because of its high sensitivity, PCR can detect HCV infection much sooner than antibody tests. Most patients have detectable levels of HCV RNA in plasma within 1 to 2 weeks of exposure. HCV RNA detection precedes ALT elevation by 10 to 12 weeks and seroconversion by 10 to 24 weeks. The highest levels of circulating viral RNA are found during the early course of infection, suggesting that patients are most infectious during this time. Variation among HCV genotypes is less likely to affect PCR test performance than antibody tests, because PCR primers are based on the highly conserved untranslated region of the HCV genome. PCR is useful in resolving ambiguous EIA/RIBA results. As many as 50% of patients who are EIA positive and RIBA indeterminate have detectable HCV RNA by PCR. Following seroconversion, PCR can be valuable in differentiating acute resolving HCV disease from chronic HCV infection in which viral RNA persists. In acute resolving HCV, RNA levels decrease rapidly, even before ALT levels. In the near future, quantitative HCV RNA tests will be available. Pretreatment levels will be particularly useful in determining which patients are the best candidates for interferon therapy. Posttreatment levels will be helpful in monitoring therapeutic response. Reference value is negative for HCV-RNA. Specimen requirement is one SST tube of blood. Tubes containing heparin cannot be used. Serum needs to be separated from cells within six hours of collection and refrigerated or frozen to avoid degradation of viral RNA. Following separation, serum can be frozen and stored for prolonged periods. |
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