Enthesitis is a recommended core domain for assessment of ankylosing spondylitis (AS), but no measurement has yet been validated according to Outcome Measures in Rheumatoid Arthritis Clinical Trials (OMERACT) criteria.
The purpose of this study was to seek to validate an enthesitis index for patients with AS according to OMERACT criteria.
An enthesitis index was validated in two AS patient cohorts: (1) a longitudinal cohort (n = 223) and (2) 22 patients from three Canadian sites participating in a 24-week randomised placebo-controlled trial of adalimumab in AS. Construct validity was evaluated by correlation analysis with the Bath AS Disease Activity Index (BASDAI), the Bath AS Functional Index (BASFI) and quality of life instruments. Reproducibility was assessed by intraclass correlation coefficient (ICC), and responsiveness was assessed by Guyatt's effect size and standardised response mean.
The most frequently affected sites were the greater trochanter and supraspinatus insertion ( approximately 20%). Patients with enthesitis had significantly greater scores for the BASDAI, BASFI, patient global, AS-specific quality of life index (ASQOL) and the Short Form 36 (SF-36) General Health Survey (p<0.001). The enthesitis score contributed significantly to variance in the BASDAI and BASFI. Interobserver ICCs were 0.96 in the longitudinal cohort and 0.89 and 0.77 in the adalimumab clinical trial cohort (for status and change score, respectively). Significant differences in change scores were evident for all patients after 24 weeks of adalimumab treatment, (p = 0.04), this being more significant when a subset of the most commonly affected entheses were analysed (p = 0.01).
AS patients with enthesitis constitute a more severe subset of disease, and the Spondyloarthritis Research Consortium of Canada (SPARCC) Enthesitis Index is feasible and reliable for measurement of this condition. Discrimination requires further study in larger trials.