Background Identifying patients with undifferentiated arthritis (UA) who are destined to develop rheumatoid arthritis (RA) is important to allow early institution of disease modifying therapy.

Objectives To describe and compare the baseline characteristics of very early UA-patients developing RA according to the 2010 ACR/EULAR RA classification criteria (UA-RA) vs. not developing RA (UA-non-RA) over a 2-year follow-up period.

Methods An early arthritis clinic was established as a prospective multi-center cohort study in 2004. Patients (18-75 years old) with ≥1 swollen joint of ≤16 weeks duration were included and followed for 2 years with examination at 0, 3, 6,12 and 24 months. A comprehensive data collection was undertaken, including a structured patient history, 66-swollen (SJC) and 28-tender joint counts (TJC), laboratory examinations and patients reported outcomes. Patients with osteoarthritis, septic arthritis or arthritis due to trauma were excluded. Mann-Whitney U-test, independent T-test and chi-square tests were used as appropriate to compare baseline characteristics between UA-non-RA and UA-RA patients.

Results 1085 patients (pts) were included from 2004 to 2010 (mean(SD) age 46.1(14.9) years, 54.7% females, median (interquartile range (IQR)) duration of joint swelling 33(12,65) days). 658 pts (after excluding pts with other diagnoses) had available data for application of the 2010 ACR/EULAR RA classification criteria at baseline, and 236/658 (35.9%) fulfilled the criteria. Of the remaining 422 UA pts, only 27 further pts (6.4%) were classified as RA according to the criteria during follow-up. Distribution of criteria points at baseline in UA-RA pts:5 points n=11, 4 points n=7, 3 points n=6, 2 points, 1 point and 0 points n=1 each. In contrast, 20/395 (5.1%), of the UA-non-RA pts had 5 criteria points at baseline (p<0.001). 13/20 (65.0%) of these pts were seronegative compared to 4/11 (36.4%) of the UA-RA pts (p=0.13). 21/27 (77.8%) of the UA-RA pts fulfilled the criteria within the first 6 months. Comparison of baseline characteristics between the two groups is shown in the table.

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Conclusions Only 6.4 % of patients with very early UA fulfilled the 2010 RA classification criteria during 2 years of follow-up, most of them fulfilled the criteria within the first 6 months. In univariate analyses female gender, smoking, seropositivity, higher joint counts and disability were associated with RA development. The number of patients with 5 criteria points at baseline was higher in the UA-RA vs. the UA-non-RA group.

Disclosure of Interest None Declared