- Sensitivity of EIA and False-Negative Results: The third-generation HCV EIA has a sensitivity of approximately 98%. [8,9,10] Circumstances associated with a false-negative EIA include patients with acute HCV infection, persons with major immune compromising conditions (advanced HIV infection or organ transplantation recipients), and persons with chronic renal failure on long-term hemodialysis.
- Specificity of EIA and False-Positive Results: The third-generation HCV EIA has a reported specificity greater than 99%; false-positive tests can occur with increased gamma globulin production, with autoimmune diseases, and following immunizations. [8] In addition, a false-positive test is more likely when performing widespread testing in populations that have a very low HCV prevalence.
Molecular HCV RNA Tests
Molecular diagnostic tests for hepatitis C specifically detect HCV RNA and the process is commonly referred to as a Nucleic Acid Test (NAT) or Nucleic Acid Amplification Test (NAAT). [18] The HCV NAT becomes positive approximately 1 to 2 weeks after initial HCV infection. [19] The NAT test has become the gold standard supplemental test for patients who have a positive HCV EIA screening test. [2,18] The NAT can determine whether a patient with a positive HCV antibody test has current (active) or resolved HCV infection. [2,6] In addition, the NAT can be used in combination with other laboratory studies, such as prior antibody test results or hepatic aminotransferase levels, to suggest the possibility of acute HCV infection. [19] The results for the commercially available quantitative HCV RNA assays, which were previously reported as copies/mL, are now given in International Units per milliliter (IUs/mL). [6]
- Qualitative HCV RNA: The qualitative HCV RNA tests provide a yes or no answer to whether detectable HCV RNA is present in the patient's blood sample. [20] The qualitative HCV RNA assays are FDA-approved for HCV diagnostic purposes. These tests, however, do not provide a quantitative level of HCV and are not used for baseline HCV RNA levels or for monitoring response to therapy. [20] For most qualitative HCV RNA assays, the lower limit of detection is 10-15 IU/mL. [20]
- Quantitative HCV RNA: The quantitative HCV RNA tests detect and quantify the number of HCV copies in the patient's blood sample, reported as IU/mL. Clinically, these tests are used for diagnosing HCV and monitoring response to therapy. Quantitative HCV RNA tests used for diagnosis and monitoring should have a lower limit of detection of 25 IU/mL or less. [6,21]
- Point-of-Care HCV RNA Assay: On June 27, 2024, the U.S. Food and Drug Administration approved the first point-of-care HCV RNA test (Xpert HCV test) to be used for qualitative detection of HCV. [22] This test can be used in clinical settings that are operating under a Clinical Laboratory Improvement Amendments (CLIA) Certificate of Waiver. [22] The test requires a blood sample obtained from a fingerstick. The blood sample is run on the GeneExpert Express System, which performs an automated in vitro reverse transcription polymerase chain reaction (RT-PCR) test. The qualitative test result (HCV detected or not detected) is available in approximately 1 hour.
Immunoassays for HCV Core Antigen
As an HCV diagnostic marker, HCV core antigen has been studied, either alone or as an HCV antibody-HCV antigen combination assay. [23,24] Some experts have proposed the use of an HCV core antigen test as a less expensive option than HCV RNA testing, but there are no HCV antigen assays (or HCV antigen-antibody combination assays) that are FDA-approved for use in the United States at this time. [25,26]
