The diagnosis of HCV infection is made by the detection of HCV antibody and on of HCV RNA by polymerase chain reaction (PCR) in serum. Detection of HCV antibody by enzyme-linked immunosorbent assay (ELISA) should be confirmed with another serological assay, usually a different ELISA. HCV RNA testing should be undertaken to establish persistence of viral replication. Although false-negative assays are uncommon with third-generation HCV ELISAs in HIV-negative people, failure to detect HCV antibody by HCV ELISA in people with HIV has been reported.11 Patients who are HCV-antibody-negative with evidence of liver disease (e.g. elevated serum transaminase levels), or a history of high-risk behaviours, should be evaluated for HCV infection with HCV RNA testing. Patients diagnosed on the basis of a positive serum HCV RNA assay alone should be confirmed by repeating the test. As the incubation period for HCV infection is between 2 and 26 weeks,12 HCV RNA testing should be undertaken in seronegative people with possible early HCV infection, as the antibody response may be delayed by several months.

HCV genotyping is important, as this may affect both the choice and duration of therapy. HCV viral load may also be measured. However, this is less important than in HIV or HBV infection. Viral load does not appear to predict disease progression, but is important when measuring treatment response.13