Validation of the 2020 Japanese classification criteria for juvenile dermatomyositis and polymyositis in a pediatric cohort in Japan

BACKGROUND: The Japanese classification criteria for dermatomyositis (DM)/polymyositis (PM), revised in 2020, are applied to both pediatric and adult populations in Japan; however, their diagnostic performance in juvenile dermatomyositis (JDM) and juvenile polymyositis (JPM) remains unvalidated. METHODS: We conducted a retrospective validation study using the same multicenter dataset as in our previous validation of the EULAR/ACR classification criteria, comprising 68 physician-diagnosed JDM/JPM and 49 non-JDM/JPM controls from seven pediatric rheumatology centers in Japan between 2008 and 2015. Sensitivity and specificity of the Japanese classification criteria were calculated using the physician's final diagnosis as the reference standard and compared with existing classification criteria. RESULTS: The Japanese classification criteria classified 58 of 68 JDM/JPM patients and none of the controls, yielding a sensitivity of 85.3% and specificity of 100%. Sensitivity was slightly lower than that of the EULAR/ACR classification criteria (89.7%), but exceeded that of Bohan and Peter's criteria and Tanimoto's criteria. Ten patients were not classified as JDM/JPM, primarily due to absent muscle weakness despite supportive MRI or biopsy findings. CONCLUSIONS: The Japanese classification criteria demonstrate excellent specificity and good sensitivity for classifying patients with JDM/JPM. Their performance supports their use in pediatric practice, particularly with expanded myositis-specific autoantibody testing.