Measurement of antibody avidity for hepatitis C virus distinguishes primary antibody responses from passively acquired antibody.

A new IgG antibody avidity test for hepatitis C virus (HCV) has been developed and was validated using sera from 12 renal dialysis patients infected with HCV. In primary HCV infection low avidity antibody (mean avidity index 24%) was detected within 50 days of seroconversion whereas in long-term infection (at least 300 days after seroconversion), the mean avidity index was high (88%); in five patients, the avidity index was shown to increase rapidly as time elapsed after primary infection, whereas immunosuppressive therapy was found to delay maturation of the immune response in two further patients. The assay was then employed to confirm that a spurious outbreak of primary HCV infection in eight bone marrow transplant patients was explicable by passive acquisition of high avidity anti-HCV after intravenous immunoglobulin therapy. It is concluded that this avidity test will have an important role in the investigation of HCV infection in patients.