INTRODUCTION

Hepatitis C virus is the primary etiologic agent of parenterally transmitted non-A, non-B (NANB) hepatitis.  The infectious agent is an RNA virus belonging to the Flaviviridae family.  Current estimates suggest that there are approximately 150 to 170,000 new cases per year in the United States.  Groups at greatest risk include IV drug users, recipients of transfusions, and health-care workers.  In random blood donors, the prevalence of HCV antibodies varies from 0.3 to 1.5 percent.  Although screening of donors and specific antibody testing have greatly reduced the incidence of transfusion-associated HCV, it is estimated that the current risk if approximately 1:3,300 units. 

Recent studies have defined clinical features of HCV infection.  It is spread primarily by the parenteral route.  Unlike Hepatitis B, sexual and maternal-fetal transmissions are uncommon.  The acute infection may be asymptomatic; less than 25 percent develop overt hepatitis.  However, chronic hepatitis occurs in 50 to 60 percent of those infected with HCV.  This is often clinically manifested by periodic increases in hepatocellular enzyme levels.  Cirrhosis develops in approximately 20 percent of those who are chronically infected; there is generally a prolonged interval (15-20 years) between initial infection and the development of this complication.  Patients with chronic HCV infection are also at increased risk for hepatocellular carcinoma.  Treatment of chronic active hepatitis due to HCV with alpha interferon may be of benefit.

SEROLOGY

The diagnosis of HCV infection primarily depends upon detecting circulating antibodies to this virus.  The first immunoassay, an EIA test which was based on detecting an antibody to a single virally associated antigen, was introduced in 1989.  In 1992, this screening assay was replaced by a more sensitive and specific second-generation EIA test, which utilizes multiple target antigens.  In addition, a recombinant immunoblot assay (RIBA-II) test has been licensed as a supplemental test following EIA positive test results.

Nevertheless, many discrepancies still exist between antibody test results and the actual presence of the virus.  The rate of false positive EIA-2 tests in a blood donor population may be as high as 70 percent.  Autoimmune conditions, recent influenza vaccination, and other causes may result in false positive HCV EIA results.  Many of these false positive tests may be resolved by supplemental RIBA-II testing.  In a subject with a positive EIA-2 test, a negative RIBA-II test strongly suggests that the former was falsely positive.  These specimens (EIA-2 pos, RIBA-II neg) are consistently negative for the presence of viral RNA.

Unfortunately, a significant number of EIA-positive specimens are not adequately resolved by RIBA-II testing.  Even in individuals with positive EIA-2 tests confirmed by supplemental RIBA-II testing.  HCV RNA may be undetectable in up to 25 percent.  These patients (EIA+ RIBA+) therefore require additional testing for the presence of viral RNA by PCR to document chronic infection.  Finally, indeterminate RIBA-II results following EIA-2 positive screening tests need to be resolved by specific testing for viral RNA.

MOLECULAR DIAGNOSTICS

The polymerase chain reaction (PCR) is an extraordinarily sensitive molecular-based diagnostic test that has been recently adapted for detecting hepatitis C viral RNA.  By reverse transcription (RT), a DNA copy of the RNA viral genome is synthesized.  This DNA copy is then amplified via the enzymatic technique of PCR.  The sensitivity of the RT-PCR reaction is approximately 1000 viral genomes/ml.

RT-PCR can be helpful in the recognition of acute HCV disease prior to seroconversion.  Following seroconversion, it can be valuable in differentiating acute resolving HCV disease from chronic HCV in which viral RNA persists.  (In resolved HCV infections, antibodies are lost very slowly.  It is estimated that the rate of antibody loss is only 0.6/100 patient years.)

In non-A, non-B chronic hepatitis, testing for HCV RNA by PCR may be helpful despite negative serology.  False negative HCV EIA tests have been identified.  Viral HCV RNA can be detected by PCR in up to 10 percent of non-A non-B hepatitis cases with negative HCV EIA.  In immunosuppressed patients, such as those undergoing organ or bone marrow transplant, false negative antibody tests appear to be a common finding.  Seroconversion occurs in only 60 to 70 percent of transplant patients who acquire HCV during transplantation.

QUANTITATIVE HCV-RNA

Once the presence of chronic HCV infection has been documented by RT-PCR, quantitation of viral load may provide clinically useful information.  Recent evidence suggests that this test may give prognostic information; lower HCV-RNA levels appear to be associated with less symptomatic disease wand with improved response rates to alpha interferon therapy.  Higher viral titers are associated with longer terms of infection indicating the progressive nature of the disease.  Patients with a higher viral loads tend to have more profound changes in liver histopathology and are more refractory to treatment.

Clinical studies indicate that interferon therapy is beneficial in approximately 50 percent of chronically infected HCV patients.  Long-term response rates (follow-up >6 months post-therapy) are lower; about 25 percent remain HCV-RNA negative.  Response rates may vary depending on dosages given.  Using quantitation, response to therapy can be monitored directly by measuring viral titer.