Maxime Verhoeven

176 Chapter 9 The concurrent (i.e., at the same time-point) correlations between DAS28 and OST- score and between SJC and OST-score were moderate (correlation coefficients ranging from 0.18 to 0.39 and from 0.35 to 0.47, respectively) and statistically significant. Lower (often) non-statistically significant correlations of OST-score were found with TJC, HAQ and EQ5D-5L, see Table 2. Non-concurrent correlations were also generally lower and often not statistically significant. Table 2 Correlation coefficients between OST-scores and DAS28/SJC28/TJC28/HAQ/ EQ5D over time. OST-score baseline month 1 month 3 month 6 DAS28, baseline 0.31* 0.25* 0.27* 0.11 DAS28, month 1 0.17 0.18 0.26* 0.11 DAS28, month 3 0.26* 0.25* 0.39* 0.39* DAS28, month 6 0.17 0.20 0.31* 0.34* SJC, baseline 0.41* 0.49* 0.42* 0.39* SJC, month 1 0.33* 0.47* 0.51* 0.50* SJC, month 3 0.23 0.30* 0.38* 0.47* SJC, month 6 0.30* 0.19 0.16 0.35* TJC, baseline 0.00 -0.02 -0.02 -0.02 TJC, month 1 -0.05 -0.04 0.07 0.08 TJC, month 3 0.16 0.17 0.31* 0.39* TJC, month 6 0.07 0.16 0.29* 0.46* HAQ, baseline 0.08 0.20 0.01 HAQ, month 3 -0.01 0.10 0.10 HAQ, month 6 -0.23 0.08 0.09 EQ5D, baseline -0.05 -0.21 -0.26 EQ5D, month 3 0.08 -0.15 -0.21 EQ5D, month 6 -0.05 -0.14 -0.35* Outcomes based on all patients (early and established), n=64. *= statistically significant OST= optical spectral transmission; DAS28= disease activity score assessing 28 joints; SJC28= swollen joint count; TJC= tender joint count; HAQ= health assessment questionnaire; EQ5D= EuroQoL-5D. The longitudinal analysis showed that one unit change in OST-score was associated with a change in DAS28 of on average 0.03 unit (95%CI 0.01–0.06). Using standardized values, this could be interpreted as a change of one SD unit in OST-score being related to a change in DAS28 of on average 0.13 SD unit (95%CI 0.03–0.23). Hence changes in DAS28 value can to some extent be estimated from the OST-score changes. This association was not modified by RA stage (p=0.96 for the interaction term). When adding OST-score to the model with only the previous DAS28 (autoregressor) and visit, the model’s ability to estimate DAS28 over time (i.e., explained variance) increased, with 2%, to 59%, Figure