Quantification of hepatitis C virus RNA: clinical applications of the branched DNA assay

Hepatitis C virus (HCV), a small, enveloped, single-stranded RNA virus, is the major causative agent for parenterally transmitted and sporadic chronic non-A, non-B hepatitis. An important public health and economic problem, a t least six main genotypes and 11 subtypes of HCV have been found with vary ing frequencies throughout the world [1]. More than 50% of individuals who become infected with HCV will develop chronic hepatitis, of whom at least 2 5% will develop cirrhosis over a 3-20 year period. A large number of these patients also will develop complications of end-stage liver disease includi ng liver failure, portal hypertension, and hepatocellular carcinoma. Since the cloning of the HCV genome, a number of nucleic acid-based assays for detection and quantification of HCV RNA have been developed to aid in t he diagnosis and monitoring of HCV infection. Assays based on target amplif ication techniques, such as reverse transcription followed by polymerase ch ain reaction (RT-PCR), show exquisite sensitivity for detection of HCV RNA in patient specimens. However, these assays yield only semi-quantitative es timates of HCV RNA content, and the possibility of obtaining false positive results with RT-PCR requires that numerous precautions be taken to avoid c ontamination of specimens with PCR products and carryover from other patien t specimens. By contrast, a quantitative assay based on branched DNA (bDNA) technology allows direct measurement of HCV RNA at physiological levels in patient specimens [2]. Since the bDNA assay quantifies HCV RNA by boosting the reporter signal, rather than replicating target sequences as the means of detection, it is not subject to the errors of target amplification tech niques. The bDNA assay is standardized to ensure accurate measurement and r esults are expressed as megaequivalents (MEq) per mi, where 1 MEq is define d as the amount of HCV RNA that generates a level of light emission equival ent to that generated by 10(6) copies of quality level 1 RNA standard [3]. Inherently quantitative and nonradioactive, the bDNA assay has been used by several groups investigating the natural history, pathogenesis and treatme nt of HCV-associated disease. The bDNA assay for quantification of HCV RNA may be a useful tool for the m anagement of patients chronically infected with HCV. A complement to serolo gical, biochemical and histological measures of disease, HCV RNA quantifica tion serves as a direct measure of the virus. Many studies have shown that serum HCV RNA levels are correlated with disease progression and clinical o utcome. Further, changes in serum HCV RNA levels in response to therapy hav e been well documented. The examples discussed here illustrate how use of t he bDNA assay to quantify HCV RNA in patient specimens may facilitate a mor e rational approach to therapy by providing clinicians with information nee ded to follow viral burden throughout the course of disease, select and adj ust treatment protocols, and evaluate the efficacy of therapeutic regimens.