Classes of participants with chronic back pain suspicious of axial spondyloarthritis identified in the 2-year latent transition analysis model (n=702)
| Class 1 ‘axial’ (p*=0.18, N†=130) | Class 2 ‘IBP+peripheral’ (p*=0.16, N†=110) | Class 3 ‘at risk’ (p*=0.29, N†=204) | Class 4 ‘no SpA’ (p*=0.37, N†=258) | |
| Sacroiliitis on MRI-SIJ (ASAS) | 0.97 | 0.22 | 0.03 | 0.01 |
| BME on MRI-Spine (≥5 lesions) | 0.09 | 0.01 | 0.00 | 0.01 |
| Definitive damage on X-SIJ (mNY) | 0.14 | 0.01 | 0.01 | 0.02 |
| ≥1 syndesmophyte on X-Spine | 0.09 | 0.19 | 0.09 | 0.09 |
| Elevated CRP (>5 mg/L) | 0.50 | 0.43 | 0.28 | 0.22 |
| Good response to NSAID ever | 0.62 | 0.71 | 0.43 | 0.33 |
| Peripheral arthritis ever | 0.11 | 0.65 | 0.02 | 0.08 |
| Dactylitis ever | 0.05 | 0.35 | 0.03 | 0.01 |
| Heel pain ever | 0.15 | 0.76 | 0.14 | 0.10 |
| Family history of SpA (ASAS) | 0.45 | 0.51 | 1.00 | 0.00 |
| Psoriasis ever | 0.10 | 0.47 | 0.08 | 0.04 |
| Uveitis ever | 0.14 | 0.11 | 0.11 | 0.06 |
| Inflammatory bowel disease ever | 0.02 | 0.08 | 0.06 | 0.10 |
| Inflammatory back pain ever | 0.81 | 0.96 | 0.72 | 0.62 |
Conditional probabilities (ie, the probability of a feature being present in one of the classes, range: 0–1) were obtained using a latent transition analysis model with full invariance on baseline and 2-year data. Full invariance means that these probabilities are the same at baseline and follow-up.
Cells are coloured in green whenever the conditional probability is ≥0.3. This cut-off was chosen to better visualise differences between the classes.
*Marginal probability of the latent class (ie, a participant's probability of class membership).
† Participants categorised to one of the classes based on their posterior probability of class membership (with the class having the highest probability for each patient determining their assignment).
ASAS, Assessment of SpondyloArthritis international Society; BME, bone marrow oedema; CRP, C reactive protein; IBP, inflammatory back pain; mNY, modified New York criteria; NSAID, non-steroidal anti-inflammatory drug; SIJ, sacroiliac joints; SpA, spondyloarthritis; X-SIJ, radiograph of the sacroiliac joints; X-Spine, radiograph of the spine.