Objective:

The objective of this study is to build a structural model visualising and quantifying the interrelationships of different disease outcomes with the Assessment of SpondyloArthritis International Society Health Index (ASAS HI) in patients with axial spondyloarthritis (axSpA).

Methods:

Cross-sectional data collected at month 72 of the Devenir des Spondylarthropathies Indifferénciées Récentes cohort was analysed. Combining prior knowledge and observed data, probabilistic Bayesian network modelling was used to study how the interplay of different disease outcomes affects the ASAS HI, which measures disease-specific overall functioning and health. Disease outcomes comprised, among others, the Ankylosing Spondylitis (AS) Disease Activity Score (ASDAS) and the Bath AS Functional Index (BASFI).

Results:

Data of 384 patients were analysed. The obtained structure suggests that ASAS HI is determined by both patient-reported physical function (BASFI) and disease activity (ASDAS). The parameters of the structural model show that an increase of ASDAS or BASFI by 1 unit corresponds to an increase of ASAS HI by 0.70 or 1.25 units, respectively. Moreover, the model suggests that disease activity has an indirect impact on ASAS HI via BASFI. No relationship between spinal mobility or structural damage and ASAS HI was found.

Conclusions:

This is the first structural model developed to better understand the construct and the interplay between clinically relevant outcomes related to ASAS HI in axSpA patients. It shows that disease activity and physical function have a strong impact on ASAS HI, confirming it to be a valid construct of overall functioning and health in axSpA patients.