Maxime Verhoeven

189 Development and validation of DAS-OST Statistical analyses Patient and disease characteristics were described using means with standard deviations (SD), medians with interquartile ranges (IQR) or frequencies with proportions, where appropriate. Model development A random sample of 2/3 of the data from the MMC cohort was used as development cohort. Using linear regression, we developed disease activity scores including OST-scores (DAS-OST). In these analyses, DAS28-ESR was used as dependent variable (i.e., reference standard) and OST-score as independent variable. The association between OST-score and DAS28 was assessed for linearity graphically as well using categorical values (defined by quartiles) and using quadratic transformations of OST-score. Covariates (i.e., ESR and VAS, and age, gender, disease duration, RF and/or anti-CCP status, which were deemed to possibly influence OST-score) were added to the model and removed one- by-one, retaining all variables that showed added predicted value beyond the OST-score (as judged by p-value ≤0.20 or a decrease in adjusted R-square upon removal). Finally, modification of the association between OST-score and DAS28 by relevant covariates (i.e., those retained in the model) was evaluated, one-by-one, retaining interaction terms with a p-value ≤0.20. OST-scores could be used as a total sum or expressed as number of joints with inflammation (JC-OST: assessing 22 joints, MCP1-5, IP1, PIP2-5, wrist, all bilaterally) as typically used in disease activity indices. Therefore, we developed a DAS-OST based on JC-OST as well. To define JC-OST, first a mixed effect logistic regression analysis (mixed effect to account for clustering of joint scores within patients) using a random intercept was performed with joint swelling (yes/no, DAS28 component) as dependent variable and OST-score of the corresponding individual joints, side (left/right), and joint type (MCP, (P) IP, wrist) as independent variables. This was done to decide whether assuming one cut-off of OST-score for swelling is appropriate for every joint or whether specific cut-offs for a joint(-type) and/or side are more appropriate. Optimal cut-off(s) were defined thereafter using Youden’s index. In line with the square root transformation of swollen joint count in DAS28, the square root of JC-OST was also used in DAS-OST development. Further a DAS-OST formula without patient VAS was developed (i.e., ‘objective index’), using the same methodology. 10 Model validation Using the formula (and applying the derived OST-score cut-offs for individual joints) as derived in the development cohort, DAS-OST was calculated in the internal validation 10