LAUSR

OBJECTIVES New onset undifferentiated large joint inflammatory arthritis can be diagnostically challenging. It is unknown how often these patients progress to rheumatoid arthritis (RA), and how they can be identified at first presentation. We assessed clinical and serological features associated with RA-development in patients with an undifferentiated mono- or oligo-articular large joint arthritis, and with keen interest if an MRI of the small joints of the hand and foot would aid diagnosis. METHODS Leiden Early Arthritis Clinic includes 4018 patients; this prospective study follows 221 consecutively included patients with new onset undifferentiated large joint arthritis. Baseline clinical data and serology were obtained. Forty-five patients had MRIs (hand and foot). MRIs were scored according to the OMERACT RAMRIS. Univariable and multivariable logistic regression were assessed. Test characteristics, predictive values and net reclassification index (NRI) for RA was determined. RESULTS Patients mostly presented with knee or ankle mono-arthritis. During the 12 months follow-up 17% developed RA. Autoantibody-positivity (ACPA and/or RF) and MRI-detected synovitis in hands and feet were independently associated with RA-development in multivariable analyses (odds ratio 10.29 (p = 0.014) and OR 7.88 (p = 0.017), respectively). Positive predictive value of autoantibodies, MRI-detected synovitis and combination of both features was 63%, 55% and 100%, respectively. The addition of MRI-detected synovitis to autoantibody-status improved diagnostic accuracy (NRI 18.1%). CONCLUSION In patients presenting with undifferentiated large joint arthritis 17% will develop RA. Autoantibody-positivity and subclinical synovitis are independent predictors. The data suggests MRI of small joints is beneficial for early identification of RA in large joint arthritis.