Diagnosis and subtyping of idiopathic inflammatory myopathies: caution required in the use of myositis autoantibodies

Detection of myositis autoantibodies (MAs) has utility in both the diagnosis and subtyping of idiopathic inflammatory myopathies (IIMs). Multiplex assays such as the Euroimmun line immunoassay (LIA) have significant limitations in rare diseases like IIM. A retrospective cohort study was performed on positive MA detected on LIA in 171 patients using the manufacturer's recommended cut‐off. Only 16.7% were deemed true positive after clinical correlation. Autoantibody‐specific cut‐offs were created and applied to the original cohort, along with generically applied higher cut‐offs. Positive predictive value (PPV) improved, but there was variable increase in false negatives. False positive MA results are common using LIA, but locally derived cut‐offs can improve performance. Clinicians must be aware of the limitations of LIA, which is the commonest method for MA detection in Australasia.