Background

Juvenile idiopathic arthritis (JIA) is a heterogeneous con-dition with time of onset before the age of 16. The diver-sity in clinical manifestations differs from that in adult arthritis, but some findings point to similarities among children and adults. Reliable biomarkers for prediction of outcome and choice of treatment in JIA are scarce [1]. JIA is currently described by the International League of Associations for Rheumatism (ILAR) classification system as a disease comprising seven categories, based on clinical characteristics, heredity and laboratory results [2]. While IgM rheumatoid factor (RF) is a determinant of the polyarticular categories, occurrence of IgA RF and anti-citrullinated peptide antibodies (ACPA), for which the most common clinical test is anti-cyclic citrullinated pep-tide (anti-CCP), are not included in the ILAR classification. In population-based studies on JIA, IgM RF has an oc