Mapping from disease-specific measures to utility: an analysis of the relationships between the Inflammatory Bowel Disease Questionnaire and Crohn's Disease Activity Index in Crohn's disease and measures of utility.

OBJECTIVES: To examine the relationship between the Inflammatory Bowel Disease Questionnaire (IBDQ), Crohn's Disease Activity Index (CDAI) and measures of utility (EQ-5D and the SF-6D indexes), and to estimate algorithms to map the two utility values from IBDQ and CDAI scores.
METHODS: A large data set from clinical trials in Crohn's disease provided contemporaneous patient responses to all four questionnaires. Paired observations from multiple time-points were analyzed. We calculated mean utility scores by IBDQ and CDAI score deciles; Spearman correlation coefficients for paired observations between IBDQ and EQ-5D (n = 3320) and IBDQ and SF-6D (n = 3230), and explored regression models using maximum likelihood estimation. The IBDQ/SF-6D model was validated against paired observations from an independent data set.
RESULTS: The IBDQ decile analysis demonstrated a consistent positive relationship with both utility indexes. Correlations between the IBDQ and both the EQ-5D and SF-6D were statistically significant (P < 0.0001), with correlation coefficients of 0.76 and 0.85, respectively. A simple linear model between EQ-5D and IBDQ explained 45% of the variance. The residuals plot for the IBDQ/SF-6D model suggested some nonlinearity and a nonlinear model explained 69% of the variance. In the validation analysis, no statistically significant difference was observed between the mean observed SF-6D and the SF-6D scores estimated using the IBDQ/SF-6D regression model.
CONCLUSIONS: Given the strength, consistency, and predictable characteristics of the relationships, the algorithms appear to provide valuable and valid methods to estimate utilities from IBDQ scores (but not CDAI) in trials of Crohn's disease patients that have collected IBDQ scores but not utilities.