For each antibody, the number of positive samples (and percentage) is given for myositis patients and controls as well as the number of positive samples with a value higher than 3 times the cut-off (for patients with myositis). The number of concordant (conc) results is given as follows: on the line that describes the Alphadia results, concordance between all three assays (first number), between Alphadia and Euroimmun (second number) and between Alphadia and Trinity (third number) is given. On the line that describes the Euroimmun results, concordance between Euroimmun and Trinity is given. The likelihood ratio (LR) is given as well as the association between each of the antibodies and IIM (evaluated by χ² testing or Fisher’s exact test (if cell size was <10) using Analyse-it for Excel). Patients with IIM (female/male: 80/64) (median age at diagnosis 53 years; age range 3–84 years) included dermatomyositis (n=57), polymyositis (n=48), antisynthetase syndrome (n=15), necrotising myositis (n=6), clinically amyopathic dermatomyositis (n=7), sporadic inclusion body myositis, overlap and undifferentiated myositis (n=9) and undefined (n=2). Diagnosis was based on a combination of clinically significant muscle weakness, elevated creatine kinase levels, electromyography, muscle biopsy (available in 90 of 144 patients) and/or skin manifestations. Demographic data (female/male, median age (age range)) of the controls were 149/91, 57 years (16–85 years); 17/23, 56 years (18–69 years) for the blood donors; 8/32, 56 years (32–75 years) for the chronic inflammatory demyelinating polyneurophathy (CIPD); 30/10, 57 years (26–75 years) for rheumatoid arthritis (RA); 25/15, 61 years (41–81 years) for systemic sclerosis (SSc); 35/5, 52 years (16–85 years) for Sjögren’s syndrome (SjS); and 34/6, 47 years (25–84 years) for systemic lupus erythematosus (SLE).

IIM, inflammatory myopathy.