Disease activity indices used in axial spondyloarthritis perform similarly in real-life clinical settings

Objective Monitoring of axial spondyloarthritis (axSpA) activity using validated indices (BASDAI/ASDAS) is widely recommended but rarely followed in practice. The reasons, although varied, may be found in the scarcity of studies comparing the performance of these indices in daily practice. Here we compare the performance of activity indices in clinical practice. Methods Observational cross-sectional study involving 330 patients. The BASDAI, ASDAS, BASFI and ASAS HI indices were included. Their correlations, degree of concordance, and discriminating capacity for different levels of activity and impact were compared using the appropriate statistics. Results A total of 127 (38.5%) women and 203 (61.5%) men were included, mean age 47.6 (SD 12.9) years, median disease duration 8 [IQR 4-16] years. At inclusion, 209 (63.3%) patients were receiving biological therapies, mostly anti-TNF. All measurement indices were highly correlated (Pearson's r ≥ 0.73). Concordance between instruments was substantial both with regard to the different activity thresholds and the different disease impact categories (k ≥ 0.61). BASDAI cutoffs of 3.95 (AUC 0.90) and 5.85 (AUC 0.90) accurately identified the ASDAS high and very high activity categories, respectively. An ASDAS ≥ 2.1 (AUC 0.87) and a BASDAI ≥ 3 (AUC 0.92) accurately discriminated the ASAS-HI high impact category. Regardless of systemic therapy use, there was substantial agreement between BASDAI remission (≤2) and ASDAS inactive disease (<1.3). Conclusion The metrological performance of standard activity indices in axSpA was similar. The BASDAI values that identify the ASDAS categories are novel. We suggest using these indices interchangeably in clinical routine.