Diagnostic performance of an immunoblot assay for anti-F-actin antibodies in type 1 autoimmune hepatitis

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Autoimmune hepatitis (AIH) is a liver disease of unknown cause characterized by hypergammaglobulinemia, typical or compatible liver histology, the absence of viral hepatitis and the production of autoantibodies. Anti-smooth muscle antibodies (SMAs) detected with indirect immunofluorescence (IIF) in rodent tissues are not disease specific, whereas antibodies directed to the filamentous form of actin (F-actin) are specifically involved in AIH-1. As solid-phase immunoassays (SPAs) specifically targeting F-actin, such as enzyme-linked immunosorbent assay (ELISA), have already been included in recent guidelines, in this study, we evaluated the diagnostic performance of an immunoblotting SPA for F-actin. We selected 86 samples positive for SMA by IIF (titre≥1∶40) and/or for anti-F-actin by ELISA (≥20 units); the patients were divided into 3 groups: AIH-1 ( n =14), other liver disorders ( n =38) and other nonliver-related conditions ( n =34). The samples were tested with an immunoblot SPA (European Autoimmune Liver Diseases 9 Ag plus F-actin, Euroimmun, Germany); the qualitative results were converted into numeric intensity values via EurolineScan software. Immunoblotting revealed 16 positive samples (19%), and ELISA revealed 24 (28%); among them, 7/16 (44%) and 11/24 (46%) had AIH-1. The diagnostic performance metrics were as follows: sensitivity (50%); specificity (87.5%); positive predictive value (PPV), 43.8%; positive predictive value (NPV), 90%; and accuracy (81.4%) and equal to those of ELISA. The mean values were greater in the AIH-1 group; the receiver operating characteristic curve (ROC) had an area under curve (AUC) of 0.77 and was not different from that of the ELISA (0.82); the agreement was 81.4%, with a Cohen’s kappa of 0.49. Immunoblotting might be a reliable assay for the identification of anti-F-actin antibodies, and given its high specificity, its implementation in a clinical laboratory might confirm the specific diagnosis of AIH-1 in patients with IIF-detected SMA.

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