Maxime Verhoeven

64 Chapter 4 lower than in the tREACH. Goodness of fit between the predicted probabilities and observed values was tested using the Hosmer-Lemeshow test, where p>0.05 indicated that a model fit the data well. All analyses were performed in R studio (Version: 3.5.3, “2019-03-11”). P-values <0.05 were considered statistically significant. Subjects with missing data (N=104) were excluded from analyses (complete-case analysis). To enhance the model’s clinical applicability and thus facilitate its clinical implementation we applied the prediction model on the combined dataset of tREACH and U-Act-Early (n=264) using the step-up approach. Therefore, the model could be simplified using fewer predictors and possible two-way interactions could be examined in a combined cohort with more power. Statistically significant interactions (p<0.05) were added to the model. To simplify the model, we assessed individual contribution of variables to the predictive power of the model by sequential addition of predictors. Model fits were compared using the likelihood ratio test. Probability for insufficient response was calculated for each patient as well as the corresponding specificity, positive predictive value (PPV), sensitivity and negative predictive value (NPV) were determined, using the “pROC” package in R. The final prediction model was uploaded into the online platform ‘Evidencio’ providing a tool for clinicians to decide whether or not to start MTX combination therapy. RESULTS Cohort comparisons In U-Act-Early (validation cohort), mean DAS28 decreased from 5.0 (± 1.1) to 3.6 (± 1.6) during the first 3 months (p<0.001). Mean DAS28 in tREACH (derivation cohort) was 5.0 (± 1.1), which decreased to a mean DAS28 of 3.1 (± 1.2, p<0.001). Baseline DAS28 in U-Act-Early was similar to that of tREACH (p=0.613), Table 1. In U-Act-Early, 58 patients (64%) were categorized as insufficient MTX responders (i.e., DAS28>3.2 at 3 months) compared to 114 (43%) in tREACH (p=0.006). In U-Act-Early, 39 (44%) patients were classified as MTX non-responders (i.e., DAS28>3.2 at 6 months), which was not significantly different from 38% in tREACH, Table 1. Additionally, U-Act-Early consisted of significantly more rheumatoid factor positive patients (81%) compared to tREACH (65%) (p=0.007), whereas no significant differences were found for anti-citrullinated protein antibody (ACPA) positivity (p=0.214), Table 1. Despite the lower folate levels due to (international) re-standardization of the method, the erythrocyte-folate levels were still significantly higher in U-Act-Early compared to tREACH (p=0.006) and genotype GG/ GA for ABCB1 was significantly more frequent in tREACH (p=0.016), while genotypes for ABCC3 were similar between cohorts, Table 1. Importantly, besides MTX, co-medication was prescribed in tREACH (derivation) but not in U-Act-Early (validation), Table 1.