Background: Objective biomarkers are needed for early diagnosis of juvenile idiopathic arthritis (JIA). Anti-A33 antibodies are considered good markers for adult rheumatoid arthritis (RA), but little information is available on their occurrence in JIA. The aim of the present study was therefore to investigate the value of anti-RA33 for diagnosis of JIA (both early and established disease), and its relation to markers of disease activity, and bone resorption.

Subjects: This case-control study was conducted on 34 children with JIA. Ten patients with arthritis of short duration (<6 weeks) were included, as undifferentiated arthritis. Forty-four age- and sex- matched healthy children served as controls. Beside evaluation and assessment of disease activity, urinary calcium, serum parathyroid hormone and serum anti-RA33 were measured in included subjects. Joints were examined radiologically and modified Larsen index (LI) was estimated.

Results: During follow up, eight of the patients with undifferentiated arthritis were diagnosed as having early JIA. Patients with JIA (early and established cases) had higher anti-RA33 levels than the control group (z = 6.04, 3.95, respectively). A total of 66.7% of the patients were positive for anti-RA33, results were comparable in early and established cases. Anti-RA33 values were correlated to disease activity (clinical and laboratory), to laboratory markers (urinary calcium, parathyroid hormone levels) and radiological evidence (LI) of bone resorption (r = 0.95, 0.63, 0.94, respectively).

Conclusion: Anti-RA33 is detected in two-thirds of JIA patients and occurs with comparable frequency early in the disease. Its levels are correlated to disease activity and markers of bone resorption and it seems to convey diagnostic and prognostic insights for appropriate management.