Validity of the bath ankylosing spondylitis disease activity index for the evaluation of disease activity in axial psoriatic arthritis

Objective To assess the validity of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) for the evaluation and definition of disease activity of axial psoriatic arthritis (PsA). Methods Fifty-four peripheral PsA, 46 axial PsA, and 103 primary ankylosing spondylitis (AS) patients were assessed. Patients were classified as having axial PsA if they had grade 2 or higher unilateral sacroiliitis in the presence of spinal symptoms. The 3 groups of patients were evaluated using several measurements for AS. Assessments of acceptability, data quality, internal consistency, construct validity, and responsiveness of the BASDAI were undertaken. Disease activity of the disease was assessed in peripheral PsA and axial PsA patients using the BASDAI, and compared with those with AS. Results For peripheral PsA patients, the Cronbach's alpha for the BASDAI was 0.783, for axial PSA patients it was 0.647, and for AS patients it was 0.786. The analysis of convergent validity showed that in peripheral PsA and axial PsA patients, the BASDAI was significantly correlated with other subjective disease activity parameters. For responsiveness, no association was found between changes in the BASDAI and changes in disease activity either in peripheral PsA or in axial PsA. BASDAI scores were similar in axial PsA and AS. Axial PsA patients with a BASDAI score >4 cm showed significant differences with peripheral PsA in terms of disease activity and were very similar to patients with AS. Conclusion The BASDAI performed similarly in evaluating disease activity in both axial and peripheral PsA. The BASDAI does not seem to be a good index for evaluating disease activity in axial PsA.