Background Familial Mediterranean fever (FMF) is a clinical diagnosis and depends mainly on disease history of recurrent attacks of serositis. A history of recurrent arthritis with erythema over the involved joint (red arthritis) is considered an important feature of FMF and used as a tool in the clinic to distinguish it from other rheumatologic conditions. Objectives The aim of this study is to determine whether history of erythema over the joint would predict FMF compared to diseased controls. Methods We surveyed patients with FMF (n=100; M, F), gout (n=16), ankylosing spondylitis (AS) (n=55), inflammatory bowel disease (IBD), (n=51) rheumatoid arthritis (RA) (n=70), Behçet’s syndrome (BS) (n=50) and systemic lupus erythematosus (SLE) (n=50) followed in our rheumatology outpatient clinics. Only those who had joint involvement were analyzed. We used a 20-item questionnaire that sought details of joint involvement. All patients were shown a picture of erythema over the joint to confirm if they had experienced a similar condition. Results Demographic features and the number of patients with a history of erythema over the joint in each group are given in the Table. Erythema over the joint predicted a diagnosis of FMF compared to overall controls which included BS, SLE, RA, AS,IBD and gout with an odds ratio of 2.5 (95% CI 1.3-4.7) (p=0.002). When the odds ratio was calculated for each control group, erythema over the joint discriminated FMF from AS OR=3.2 (95% CI 1.2-9.0) (p=0.04) and RA OR=4.9 (95% CI 2.3-10.8) (p=0.001). If history of red arthritis with an onset before the age of 20 is considered, an OR of 22.8 (95% CI 8.8-61) is obtained, giving a sensitivity of 79.4% and a specificity of 85.4% for the diagnosis of FMF Table 1. Demographic characteristics and erythema over the joint frequency among the study groups Erythema over the joint,Erythema over the joint with an onset n (%)before the age of 20 FMF (n=57/100)37 (64.9)31 (54.3) 31/37 (83.7) RA (n=70/70)19 (27.14)3 (4.2) 3/19 (15.7) BD (n=23/50)13 (56.5)4 (17.3) 4/13 (30.7) SLE (n=25/50)13 (52)0 Gout (n=16/16)12 (75)0 AS (n=22/55)8 (36.3)1 (4.5) 1/8 (12.5) IBD (n=5/51)3 (60)0 Conclusions History of erythema over the joint (red arthritis) may discriminate FMF arthritis from certain arthritidies. The sensitivity and specificity improves significantly if age of onset of red arthritis is taken into consideration. This observation should be improved by including a control group consisting of other periodic fever syndromes. Disclosure of Interest None Declared