POS0650 FACING THE ISSUES OF CHANGE IN ENTRY VISIT DIAGNOSIS AND LOST OF FOLLOW-UP OVER TIME IN INCEPTION COHORTS: DATA FROM THE AXIAL SPONDYLARTHRITIS (AxSpA) DESIR COHORT

Background Inception cohorts can be defined as long term follow-up of patients suffering at entry visit from a recent onset disease. Despite their huge advantage to better approach the truth in terms of long term prognosis of the disease, they are usually facing 2 issues: a) the possibility of a change in the entry visit (initial) diagnosis b) the missing data due to patients lost of follow-up. Objectives To evaluate both the frequency and the baseline predisposing factors of a) a change in the initial diagnosis b) the risk of loss of follow-up in the DESIR axSpA cohort. Methods Study design :DESIR is an ongoing (10 year follow-up completed for all the patients) multicenter cohort of recent onset axSpA. Diagnosis : At entry visit and during the 10 year follow-up period, this diagnosis was based on the opinion of the treating rheumatologist with the requirement of a highly suspected diagnosis of axSpA at entry visit and after the first 2 years of follow-up the possibility to exclude the patients in case of a change in the initial axSpA diagnosis. Statistical analysis : Multiple imputation was used to address missing data and estimates the probabilities of a change in initial axSpA diagnosis for each patient lost of follow up. Predisposing factors of an unchanged initial axSpA diagnosis were then evaluated using a multivariate logistic regression model on imputed data sets. A multivariate cox survival analysis exploring factors associated to the overtime risk of loss of follow-up was also performed. Results Of the 708 enrolled patients, 45 were excluded from the cohort because of a documented change in the initial axSpA diagnosis (mechanical low back pain n = 30, fibromyalgia n = 13, chronic inflammatory rheumatic disease (n = 1 and no information n = 1).The classification criteria were fulfilled by 16/45 (36%) versus 413/663 (63%) and 21/45 (47%) versus 522/663 (81%) patients with a documented change versus no change in their entry visit diagnosis according to the ASAS ax-SpA and the AMOR criteria respectively. During the 10 year follow-up period, 300 patients were lost of follow-up. Based on imputation, among these 300 patients, 19 patients were systematically suspected to have a change in their initial axSpA diagnosis in all imputations while 173 patients were never “suspected” for this change; 42 patients were considered as suspected for a change in their initial axSpA diagnosis in at least 70% of imputations. Predisposing factors of an unchanged initial axSpA diagnosis were (odds ratio [95% CI]): radiographic SIJ structural damage: 17.0 [4.1; 71.0]; past or present psoriasis: 5.3 [2.0; 14.3]; CRP ≥ 6 mg/l: 2.7 [1.3; 5.3]; good response to NSAID: 2.5 [1.5; 4.2]; HLA B27 positivity: 2.0 [1.3; 3.3]; anterior chest wall pain: 2.0 [1.2; 3.3] and female gender: 1.9 [1.2; 3.0]. Predisposing factors of the risk of loss of follow-up were: Age at back pain initiation< 45 years old: 1.8 [1.2; 2.9]; CRP < 6 mg/l: 1.5 [1.1; 1.9]; HLA B27 negativity: 1.4 [1.1; 1.8]; educational level < university: 1.4 [1.1; 1.8]; smoker: 1.3 [1.0; 1.6]. Conclusion These data suggest that a) a change in the entry visit diagnosis and the risk of follow-up have to be considered in inception cohorts b) statistical models including multiple imputations could facilitate the evaluation of long term prognosis of the disease. Acknowledgements The authors would like to thank the investigators of the 25 centers and all the 708 enrolled patients. This study has been supported via unrestricted grants from Pfizer france and the French society of Rheumatology. Disclosure of Interests None Declared.