A new set of criteria for classifying adult and juvenile idiopathic inflammatory myopathies (IIM), which inflame the skeletal muscles, tends to outperform prior classifications. So write Ingrid Lundberg, of Karolinska University Hospital in Stockholm, Sweden, and colleagues in Arthritis & Rheumatology of the new classification, which the European League Against Rheumatism and the American College of Rheumatology jointly developed.

IIM may occur at any age, but it tends to affect adults between the ages of 40 and 60, and children aged five to 15, according to the U.S. National Library of Medicine. Common forms of the disorder are polymyositis, dermatomyositis, and sporadic inclusion body myositis.

The new criteria classify IIM in terms of “definite,” “probable,” and “possible,” and international rheumatology, dermatology, neurology, and pediatric groups have endorsed them, the scholars write. They drew upon data from 976 idiopathic inflammatory myopathy patients, as well as 624 patients who had mimicking conditions but not IIM.

“Compared with most previous criteria, the new criteria are superior in sensitivity, specificity, and classification accuracy,” writes Mariah Zebrowski Leach in Rheumatology Network. She adds that a potential limitation was an oversampling of caucasian patients, and the classification, thus, still requires validation among Asian and African patients.