Objective To externally validate the performance of the new European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria set for idiopathic inflammatory myopathies (IIM) with a Japanese cohort.
Methods This study included 420 IIM and 402 non-IIM cases. Probability of having IIM in each patient was calculated using the collected data set. The cut-off probability was set at 55%, as recommended by EULAR/ACR. Patients classified as IIM by the criteria were further subclassified with classification trees.
Results When the probability cut-off was set at 55%, the sensitivity/specificity of the new criteria to diagnose IIM were 89.3%/91.0% in the total cohort, 88.1%/95.1% without muscle biopsy data and 90.4%/65.5% with biopsy data. The cohort included 12 overlap syndrome patients with biopsy data, who were included as non-IIM cases in accordance with traditional Japanese methods. When they were included in the IIM cases, the specificity in patients with biopsy increased to 74.4%. The sensitivity/specificity of the new criteria to diagnose polymyositis/dermatomyositis (PM/DM) plus juvenile and amyopathic DM in the Japanese cohort was 87.4%/92.4%, which were greater than those of the Tanimoto’s criteria revised to enable classification of amyopathic DM (ADM) (71.2%/87.8%) and were comparable with those of Bohan & Peter’s criteria to diagnose those diseases except for ADM (88.4%/88.3%).
Conclusions Our study externally validated high specificity of the new criteria for the first time, although with several limitations, including low percentage of child patients. The new criteria have higher sensitivity and/or specificity in classification of PM/DM than the previously reported criteria, demonstrating its usefulness for interethnic patients.
- autoimmune diseases
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Handling editor Josef S Smolen
Contributors MJ, AO, SI and HK performed the research, analysed the data and wrote the paper. Other authors contributed to collect patients’ data. MJ and AO equally contributed to this work.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient and public involvement statement The clinical data of patients were retrospectively collected from the medical records using questionnaires.
Patient consent for publication Not required.
Ethics approval The project was approved by research ethics committees of individual institutes (No. 1580).
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement All data relevant to the study are included in the article or uploaded as supplementary information.
Author note Authors’ affiliations listed were current at the time that the data collection was performed. The current address of Hajime Sano is: Department of Rheumatology, Kyoto Okamoto Memorial Hospital, Kyoto, Japan.