Our objectives in this study were to compare results obtained by an enzyme immunoassay (EIA) for F-actin antibody (FAA) immunoglobulin G (IgG) to those determined by an indirect fluorescent antibody (IFA) assay for smooth muscle antibody (SMA) IgG, and to determine the prevalence of FAA in patient sera having serologic evidence of acute viral hepatitis. Sera from 415 patients suspected of having autoimmune hepatitis (AIH), 208 patients suspected of having acute viral hepatitis A, B, or C, and 100 healthy blood donors (HBD) were included in the study. Only one of 100 HBD showed low levels (20-30 Units) of F-actin IgG. In patients suspected of having AIH, the prevalence of FAA increased as SMA titers increased and all sera with SMA titers of >or=1:160 were FAA-positive. In contrast, there were many sera with negative (<1:20) or low (1:20-1:40) SMA titers that contained moderate to high levels (>30 Units) of FAA; many exceeding 80 Units. Moreover, 51.4% of these sera were also positive for anti-nuclear antibody (ANA), which is also utilized in diagnosing type 1 AIH. FAA was detected in 25% of viral hepatitis antibody-positive sera, with the majority (59.3%) containing low levels, and all were ANA-negative.
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