Establishment of the minimum clinically important difference for the bath ankylosing spondylitis indices: a prospective study.

OBJECTIVE: The minimum clinically important difference (MCID) is the minimum level of change of an outcome measure that is considered to be clinically relevant. This prospective study was conducted to establish the minimum level of change in the Bath Ankylosing Spondylitis (BAS) indices that is meaningful for both the patient and the clinician.
METHODS: The BAS questionnaires [i.e., Bath Ankylosing Spondylitis Functional Index (BASFI); Bath Ankylosing Spondylitis Disease Activity Index (BASDAI); and Bath Ankylosing Spondylitis patient Global score (BAS-G)] were administered to 125 patients with ankylosing spondylitis (AS) at baseline and at the end of a 2-week intensive physiotherapy program. Together with the final assessment, a global validated rating scale was used to examine each domain. Receiver operating characteristic (ROC) curves were plotted to determine the BAS change score that most accurately classified patients with respect to a clinically meaningful change. This analysis was repeated to investigate whether the estimate of change was dependent upon the patients' baseline scores.
RESULTS: According to analyses of ROC curves, the MCID are 7 mm or 17.5% for BASFI: sensitivity = 0.60/specificity = 0.85; 10 mm or 22.5% for BASDAI: sensitivity = 0.65/specificity = 0.82; and 15 mm or 27.5% for BAS-G: sensitivity = 0.61/specificity = 0.74. MCID values were independent of the patients' baseline scores (p = 0.09 to 0.72) by regression analysis.
CONCLUSION: This prospective study gives MCID values for BASFI, BASDAI, and BAS-G that allow translation from the BAS indices to readily understood values for both patients and clinicians.