Interlaboratory comparison of immunoprecipitation-based testing for myositis-specific autoantibodies

Myositis-specific autoantibodies (MSAs) are clinically important biomarkers that help classify idiopathic inflammatory myopathies and guide clinical management, but reliable detection remains challenging. Immunoprecipitation (IP) is widely regarded as a reference method for MSA detection in specialist laboratories. However, IP comprises a range of related techniques rather than a single standardised assay, and variability between expert laboratories using different IP-based workflows is not well characterised.

Serum samples previously submitted to the Bath Diagnostic Serology Service were distributed to three collaborating reference laboratories. Centres analysed samples using their routine workflows, which included radiolabelled protein IP, RNA-IP, IP-western blotting, IP-mass spectrometry, and supplementary immunoassays. Results were interpreted within each laboratory's diagnostic framework, and interlaboratory concordance of MSA detection and specificity assignment was evaluated.

Thirty-two samples were included. Overall concordance across overlap groups ranged from 63-78%. Agreement varied by MSA specificity and methodological scope. Anti-Zo, anti-NXP2 and anti-TIF1γ demonstrated the highest reproducibility, whereas anti-PL-7, anti-Jo-1, anti-EJ, anti-Ha and anti-HMGCR were identified only by isolated laboratories. Differences were consistent with variation in IP method, including antigen source, detection modality and availability of reference sera. Complementary assays played an essential role in anti-MDA5, anti-HMGCR and anti-NXP2 antibody detection.

These findings demonstrate that IP-based testing should be considered a framework of expert diagnostic practice rather than a single uniform assay. Greater transparency in laboratory methods and reporting practices may improve interpretation of autoantibody results and support more consistent comparison of MSA results across clinical and research settings.