OBJECTIVES: To develop and validate the cut-offs in the Juvenile DermatoMyositis Activity Index (JDMAI) to distinguish the states of inactive disease (ID), low disease activity (LDA), moderate disease activity (MDA) and high disease activity (HDA) in children with juvenile dermatomyositis (JDM). METHODS: For cut-off definition, data from 139 patients included in a randomised clinical trial were used. Among the six versions of the JDMAI, JDMA1 (score range 0-40) and JDMAI2 (score range 0-39) were selected. Optimal cut-offs were determined against external criteria by calculating different percentiles of score distribution and through receiver operating characteristic curve analysis. External criteria included the modified Pediatric Rheumatology International Trials Organization (PRINTO) criteria for clinically ID in JDM (for ID) and PRINTO levels of improvement in the clinical trial (for LDA and HDA). MDA cut-offs were set at the score interval between LDA and HDA cut-offs. Cut-off validation was conducted by assessing construct and discriminative ability in two cohorts including a total of 488 JDM patients. RESULTS: The calculated JDMAI1 cut-offs were ≤2.4 for ID, ≤6.6 for LDA, 6.7-11 for MDA and >11 for HDA. The calculated JDMAI2 cut-offs were ≤5.2 for ID, ≤8.5 for LDA, 8.6-11.3 for MDA and >11.3 for HDA. The cut-offs discriminated strongly among disease activity states defined subjectively by caring physicians and parents, parents' satisfaction or non-satisfaction with illness outcome, levels of pain, fatigue, physical functional impairment and physical well-being. CONCLUSIONS: Both JDMAI1 and JDMAI2 cut-offs revealed good metrologic properties in validation analyses and are, therefore, suited for application in clinical practice and research.

1st Faculty of Medicine Charles University General University Hospital Prague Prague Czech Republic

Department of Neurosciences Rehabilitation Ophthalmology Genetics and Maternal Infantile Sciences University of Genoa Genova Italy

Department of Paediatrics and Adolescent Medicine Faculty of Medicine Pavol Jozef Šafárik University in Košice Kosice Slovakia

Direzione Scientifica IRCCS Istituto Giannina Gaslini Genoa Italy

Division of Rheumatology Ospedale Pediatrico Bambino Gesu Rome Italy

Pediatric Rheumatology and Immune inflammatory diseases and KU Leuven University of Leuven Department of Microbiology and Immunology Laboratory Immunobiology UZ Leuven Universitair ziekenhuis Leuven Belgium

Pediatric Rheumatology Unit Aarhus University Hospital Aarhus Denmark

Servicio de Reumatología Hospital Civil de Guadalajara Unidad Hospitalaria Fray Antonio Alcalde Guadalajara Mexico

Thessaloniki University School of Medicine 1st Department of Pediatrics Pediatric Immunology and Rheumatology Referral Center Hippokration General Hospital Thessaloniki Greece

UOC Reumatologia e Malattie Autoinfiammatorie IRCCS Istituto Giannina Gaslini Genova Italy

UOC Servizio Sperimentazioni Cliniche Pediatriche IRCCS Istituto Giannina Gaslini Genoa Italy

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