Objectives To develop data-driven criteria for clinically inactive disease on and off therapy for juvenile dermatomyositis (JDM). Methods The PRINTO database contains 275 patients with active JDM evaluated prospectively up to 24 months. Thirty-eight patients off therapy at 24 months were defined as clinically inactive and included in the reference group. These were compared with a random sample of 76 patients who had active disease at study baseline. Individual measures of muscle strength/endurance, muscle enzymes, physician's and parent's global disease activity/damage evaluations, inactive disease criteria derived from literature, and other ad hoc criteria, were evaluated for sensitivity, specificity, and Cohen's kappa agreement. Results The individual measures that best characterised inactive disease (sensitivity and specificity >0.8 and Cohen's K>0.8) were manual muscle testing (MMT)≥78, physician global assessment of muscle activity=0, physician global assessment of overall disease activity (PhyGloVAS) ≤0.2, the Childhood Myositis Assessment Scale (CMAS) ≥48, the Disease Activity Score (DAS) ≤3 and the Myositis Disease Activity Assessment Visual Analogue Scale (MYOACT) ≤0.2. The best combination of variables to classify a patient as being in a status of inactive disease on or off therapy is at least 3 out of 4 of the following criteria: creatine kinase ≤150, CMAS≥48, MMT≥78, and PhyGloVAS≤0.2. After 24 months, 30/31 (96.8%) patients were inactive off therapy and 69/145 (47.6%) were inactive on-therapy. Conclusion PRINTO established data-driven criteria, with clearly evidence-based cut off values, to identify JDM patients with clinically inactive disease. These criteria can be used in clinical trials, in research and in clinical practice.

Significance and Innovations:1. Juvenile systemic sclerosis (jSSc) patients demonstrate significant differences between dcjSSc and lcjSSc subtype regarding frequency of skin, vascular, pulmonary and cardiac involvement.2. Physician global assessment of disease activity and damage is higher in the dcjSSc group.3. jSSc patients with overlap features are not 'protected' from major internal organ involvement and have a higher frequency of lung disease compared to those without overlap features.

The hMC was found to possess good measurement properties in a large population of patients with a wide range of disease activity and severity. The new tool, which is primarily intended for use in routine clinical care, should be further tested in other populations of patients evaluated prospectively.

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