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Background Juvenile dermatomyositis (JDM) is a rare autoimmune disease known to primarily cause rash and muscle weakness. The evolution of the disease is still unclear, in particular disease activity based on patient-reported outcomes. A cohort of 493 patients with 3,625 visits up to 5 years since diagnosis was used to explore disease trajectories based on the patient-reported outcome, patient/parent visual analogue scale (VAS), completed by the appropriate person depending on the child’s age. Age at diagnosis, sex, ethnicity and baseline physician's global assessment (PGA) measurements were considered as predictors of disease activity. In addition to this 8 baseline clinical/medical history variables were also considered as potentially predictive: ulcerations, Gottron’s papules, myalgia, fever, fatigue, dysphagia, respiration and gastrointestinal problems. Methods A mixed effects model was fitted to the data to identify the strongest predictors of disease activity accounting for correlations of patient/parent VAS measurements within patients. Growth mixture models were used to identify subgroups of patients that shared similar trajectories (latent classes) and logistic regression was used to predict the probability of belonging to the subgroup that had more severe disease activity. The identified latent classes of disease activity, based on the patient-reported outcome of patient/parent VAS, were compared with previously identified latent classes derived from PGA as the outcome measure. Results The results from fitting a mixed effects model showed that disease activity had a cubic relationship with time since diagnosis. Being non-white and having a history of myalgia and gastrointestinal problems was shown to predict higher disease activity across the whole follow-up time. The results from fitting growth mixture models led to identifying two classes: the first showed an improvement in condition after the first year, which correlated with results from the mixed effects model, the second, more severe class, was on average higher and showed little improvement across the 5 years. In addition to the predictors identified in the mixed effects model, skin ulceration and older than the mean age (8.3 years) at diagnosis were shown to be associated with the probability of belonging to the more severe class. Conclusion Comparing these results to those previously found in analyses of PGA data collected on the same patients, we found that the patterns of activity were similar although on average higher, indicating that reports of disease activity by patients/parents were worse than those collected from physicians. This could be due to factors influencing patient’s experiences that are not measured by physicians. Discussions with clinicians suggest that this could be due to symptoms that are difficult to measure and that are unaffected by treatment, for example, symptoms causing damage. These are often overlooked in physician’s assessments, despite being an important factor for patients. Disclosures P.J. Baptiste None. L.R. Wedderburn None. C.T. Deakin None. B.L. De Stavola None. E. Njagi None.

O17 Modelling longitudinal patient-reported outcome measures in JDM