Clinical utility of the ASDAS index in comparison with BASDAI in patients with ankylosing spondylitis (Axis Study)

The objective of the study was to study the clinical utility of the Ankylosing Spondylitis Disease Activity Score (ASDAS) for the assessment of disease activity in ankylosing spondylitis (AS) patients, compared to the Bath Ankylosing Spondylitis Activity Index (BASDAI). This was a prospective longitudinal observational study in patients with AS (NY-modified criteria) from 23 Spanish centers. Physical and analytical data; global, lumbar, and nocturnal pain; ASDAS, BASDAI and minimally acceptable clinical status (PASS) were collected. Psychometric characteristics of both indexes were analyzed: construct validity (convergent and divergent), discriminant capacity, criterion validity (global physician and patient assessment), and sensitivity to change. The study involved 127 patients (19.7% attrition). Both BASDAI and ASDAS showed a higher correlation with patient's global assessment (r = 0.76 and 0.70, respectively) than with physician's global assessment (r = 0.67 and 0.57). Both scores allowed discriminating patients with an acceptable clinical status, although BASDAI to a greater extent than ASDAS (Cohen δ 1.72 vs 0.88 for the medical PASS). Both scores showed sensitivity to change in patients who changed from an unacceptable symptomatic state to acceptable according to PASS criteria (physician and patient) and by BASDAI 50 response criteria (Cohen δ > 0.80). BASDAI showed better criterion validity than ASDAS, both for the patient PASS (AUC 0.85 vs 0.79) and for the physician's (AUC 0.90 vs 0.79). ASDAS shows adequate performance for disease activity in patients with AS; however, in this study, its psychometric properties do not present advantages over BASDAI in terms of criterion validity, sensitivity to change or discriminative capacity; replacement of BASDAI by ASDAS is not supported by the data.