- Last Update On : 2013-11-25
Hepatitis C (HCV) is a major cause of chronic liver disease in the United States. Approximately 4 million people in the United States are currently infected and an estimated 40,000 new infections are suspected each year. The initial diagnosis of HCV infection is based on positive HCV antibody tests and the detection of HCV RNA by qualitative PCR.
The initial test for hepatitis C (HCV) infection is an immunoassay that detects antibodies to multiple HCV proteins. Supplemental or confirmatory testing is recommended for all reactive HCV antibody tests to determine the presence of active infection. Detection of HCV RNA in the blood by polymerase chain reaction (PCR) is indicative of active infection. 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. 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.
Once HCV infection has been established, quantitative HCV PCR (viral load) can provide prognostic information. Treatment response has been shown to be greater for patients with low levels of HCV RNA. Most HCV infected patients should have viral load testing performed prior to treatment unless there are clear contraindications to antiviral therapy. The HCV positive patient with negative viral load should have the test repeated in 3 to 4 months, because some patients with active infection have intermittently undetectable viral loads. Predictors of response to antiviral therapy include viral load of less than 2,000,000 IU/mL, genotype other than 1, shorter duration of infection, female gender, and low body weight.
In May 2011, FDA approved two new Direct Acting Antiviral (DAA) drugs for treatment of patients with HCV genotype 1 who are treatment-naive or previous non-responders to conventional therapy. Clinical trials for these new drugs, boceprevir and telaprevir, utilized a Roche Cobas TaqMan HCV version 2.0 test to measure viral load and assess treatment response. The assay has a lower limit of quantitation (LOQ) of 25 IU/mL and lower limit of detection (LOD) of 15 IU/mL for HCV genotype 1. This compares to an LOQ of 43 IU/mL and LOD of 7-13 IU/mL for genotype 1 with the previous (v1.0) Roche HCV viral load assay.
An upgrade to this assay was implemented in July 2013 that necessitated a change in reporting at the lower limit. With the new version of the assay, the lower limit of detection (LOD) and lower limit of quantitation (LOQ) are the same at 15 IU/mL
Samples having HCV target detected, but below the quantifiable limit are reported as <15 IU/mL Samples having no HCV target detected are reported as not detected. Samples having detectable and quantifiable HCV target are reported as the numeric value up to or >50,000,000 IU/mL.
Specimen requirement for HCV Quantitative is 2 mL of serum or EDTA plasma. Tubes containing heparin cannot be used. Specimen must be centrifuged for 18 to 20 minutes and separated from the cells within 6 hours of draw.