Comparative evaluation of immunoserological detection of F-actin antibodies

Autoantibody detection remains essential in diagnosing autoimmune liver diseases (AILD), particularly autoimmune hepatitis (AIH), in which smooth muscle antibodies (SMA) targeting filamentous actin (F-actin) are commonly observed. This study assessed the concordance between immunofluorescence testing (IFT), the gold standard for SMA detection, and a commercially available F-actin ELISA in a real-world clinical setting. During the three-months study period, 244 samples were sent to our laboratory for autoantibody testing; 241 were included in the study. SMA IFT on rodent stomach, kidney and liver sections identified 104 positive cases, with ELISA-detected F-actin antibody levels being significantly higher in these cases. F-actin ELISA concentrations significantly increased with higher SMA IFT titer. Concordance between the ELISA (using manufacturer-recommended cut-offs of 20 and 30 units) and IFT depended on the SMA staining pattern. Only slight agreement was found between positive kidney SMA staining patterns and ELISA concentration. Stomach SMA staining showed moderate agreement (Cohen's kappa = 0.49) and a subgroup analysis revealed better agreement in autoimmune disease cases, particularly AIH. An optimized ELISA cut-off of 27.6 units improved agreement slightly (Cohen's kappa = 0.55). The results suggest that, although there is a certain level of concordance between the ELISA and stomach SMA IFT, the F-actin ELISA does not reliably predict SMA IFT staining. These findings underscore the importance of method validation and context-specific interpretation when implementing ELISA-based autoantibody screening in routine diagnostics.