Background Immunosuppressive medications are often stopped prior to arthroplasty to mitigate infection risk, but this may increase the risk of disease flares post surgery in patients with rheumatoid arthritis (RA). Objectives To describe rates, characteristics, and risk factors for flare after total hip (THA) and total knee (TKA) arthroplasty surgery. Methods Pre- and post-operative characteristics were examined in 58 RA patients undergoing TKA/THA. Perioperative medication use was standard of care: biologics were stopped before surgery, while steroids and methotrexate (MTX) continued. Clinicians evaluated RA clinical characteristics, on average, 0–2 weeks before and 6 weeks post-surgery. Post-surgery, patients completed weekly questions about RA symptoms, impact, and flare status using the OMERACT Flare Questionnaire. Baseline characteristics were compared using t-tests and chi-square, and multivariate logistic regression was used to identify baseline characteristics associated with post-surgical RA flares. Results Of 68 patients, 10 (15%) were flaring prior to surgery and were excluded. 88% met 2010/1987 RA criteria; those who did not meet criteria were included by rheumatologist diagnosis. Patients had a mean [SD] age of 61 [12], BMI of 30.6 [7.2], and RA duration of 16 [12] yrs. 59 (87%) were female, 53 (78%) were white, 33 (49%) were having THA, and 35 (52%) were on biologics. 35 (60%) had flared by 6 weeks post surgery. At baseline, flarers had significantly (p<.05) higher BMI, higher disease activity indicators (DAS28, RAPID3), inflammatory markers (ESR, CRP), and pain, and more were undergoing THA and used biologics (Table 1). After controlling for age, surgical joint, and baseline DAS28, the odds of flaring by 6 weeks post-surgery were significantly higher in patients who had discontinued biologics (OR 14.9, 95% CI 2.0, 112.0) or were obese (OR 6.0, 95% CI 1.1, 33.0).Table 1. Characteristics of RA patients who did and did not flare in the first 6 weeks post hip or knee arthroplasty (N=58) Flare (n=35) No Flare (n=23) Significance Age (yrs) 58.2 (13.5) 63.9 (10.4) 0.073 Female Sex 32 (91%) 18 (78%) 0.155 BMI 32.6 (7.2) 28.6 (6.5) 0.040 Minority Race 10 (29%) 2 (9%) 0.099 Hip arthroplasty 22 (63%) 6 (26%) 0.006 HOOS 39.8 (21.9) 52.2 (22) 0.230 KOOS 44.0 (27.5) 44.9 (14.2) 0.914 RA Duration (yrs) 16.4 (12.0) 16.1 (13.2) 0.920 DAS28 4.1 (1.2) 2.9 (1.3) 0.001 RAPID3 16.0 (4.1) 13.2 (5.4) 0.032 ESR (mm/hr) 23.9 (20.2) 11.0 (11.2) 0.003 CRP (mg/dl) 2.3 (3.6) 0.9 (0.8) 0.043 MD Global 4.1 (1.6) 3.3 (1.9) 0.092 Patient Global 5.1 (1.9) 4.0 (2.5) 0.064 Pain (10mm VAS) 7.0 (2.0) 5.7 (2.7) 0.053 MD HAQ 3.9 (1.6) 3.5 (1.5) 0.282 Biologics 24 (69%) 7 (30%) 0.004 MTX 20 (57%) 14 (61%) 0.778 Data shown as mean (SD) or n (%). Conclusions Flares are frequent in RA patients undergoing arthroplasty, particularly THA. Discontinuing biologics and obesity significantly increased the risk of flaring post-arthroplasty. Acknowledgement This study was supported by the Clinical Translational Science Center (CTSC) (UL1-TR000457–06). Disclosure of Interest None declared