Receiver operating characteristic analysis of the PM1-α ELISA. Results obtained from three centers and based on 567 patients including polymyositis/scleroderma (PM/Scl) patients (n = 40), Scl patients (n = 205) and PM patients (n = 40) as well as other controls were used to calculate a receiver operating characteristic analysis (a) for all control samples and (b) for unrelated controls (without Scl and PM). The curve shows a clear discrimination between PM/Scl patient samples and various controls as emphasized by an area under the curve value of 0.901 (all controls) and 0.958 (unrelated controls). The differentiation between PM/Scl patients and controls was significantly improved when Scl patients and PM patients were excluded from the control group (b). SE, standard error.