The presence of rheumatoid factor in sera from anti-HCV positive blood donors interferes with the detection of HCV-specific IgM

Background/Aims: IgM anti-HCV has been reported as a useful marker of disease activity and of likely response to interferon therapy in patients with chronic hepatitis C virus infection, Sera from patients with hepatitis C virus infection may contain rheumatoid factor, This study was designed to investigate the possible effect of rheumatoid factor in an IgM anti-HCV assay. Methods: Sera from 75 blood donors with chronic hepatitis C virus infection were tested for the presence of IgM anti-HCV using a core-derived peptide as antigen, before and after removal of IgG. These sera and appropriate control sera were also tested for the presence of rheumatoid factor. Results: Removal of IgG prior to IgM testing resulted in a significant loss of reactivity in the IgM anti-HCV assay in 30/41 sera that were positive in this assay when tested untreated, Seventy per cent of anti-HCV positive sera were also positive for rheumatoid factor. Conclusions: Rheumatoid factor causes significant false positive reactivity in IgM anti-HCV detection assays. The results of assays that do not account for this interference should be viewed with caution.