Change in different classes of chronic back pain suspicious of axial spondyloarthritis: a latent transition analysis of the SPACE cohort.

Objectives: To follow up four previously identified classes 'pure axial spondyloarthritis' (axSpA) ('axial'), 'axSpA with peripheral signs' ('inflammatory back pain+peripheral'), 'axSpA at risk' and 'no spondyloarthritis' ('no SpA'). They reflect the expert-opinion-free construct or 'Gestalt' of chronic back pain suspicious of axSpA. The aim was to assess participants' transitions between these classes over time.

Methods: Participants with chronic back pain of ≤2 years duration, suspicious of axSpA from the SPondyloArthritis Caught Early cohort were analysed. Latent class (LCA) and latent transition analysis (LTA) using clinical, laboratory and imaging data at baseline and 2 years were calculated. Conditional and marginal probabilities were obtained, reflecting the probability of a spondyloarthritis feature in a class and the probability of the participant's class membership, respectively. Transitional probabilities were extracted revealing potential switches across classes. The analyses were performed in all participants using imputations for missing data and in participants with full data at baseline and 2 years.

Results: Baseline and 2 years LCA models were constructed for 702 participants, resulting in the same four-class model as previously described. LTA revealed only a 3% transition from the 'no SpA' to the 'at-risk' class from baseline to 2 years with all other participants remaining in their initially assigned class. Sensitivity analysis on 384 participants with complete data at both baseline and 2 years showed similar results, underlining the model's robustness.

Conclusions: Transitions between the four classes over 2 years were basically inexistent, highlighting the unlikelihood of developing new class-defining features of axSpA after an initial clinical workup.