Observed baseline characteristics per latent class (n=702) after data imputation
| Characteristic | Class 1 ‘axial’ (n=130, 18%) | Class 2 ‘IBP+peripheral’ (n=110, 16%) | Class 3 ‘at risk’ (n=204, 29%) | Class 4 ‘no SpA’ (n=258, 37%) |
| Age, years | 28 (7) | 31 (8) | 30 (8) | 32 (8) |
| Male | 77 (59%) | 46 (42%) | 58 (28%) | 90 (35%) |
| Body mass index (kg/m2) | 24 (4) | 25 (6) | 24 (5) | 25 (5) |
| Duration of back pain (months) | 14 (7) | 12 (6) | 13 (7) | 14 (7) |
| Imaging features of SpA | ||||
| Sacroiliitis on MRI-SIJ (ASAS) | 125 (96%) | 14 (13%) | 0 (0%) | 1 (0%) |
| Definitive damage on X-SIJ (mNY) | 15 (12%) | 1 (1%) | 3 (1%) | 5 (2%) |
| BME on MRI-Spine (≥5 lesions) | 12 (9%) | 1 (1%) | 0 (0%) | 2 (1%) |
| ≥1 syndesmophyte on X-Spine | 9 (7%) | 21 (19%) | 14 (7%) | 20 (8%) |
| Clinical features of SpA | ||||
| Peripheral arthritis ever | 13 (10%) | 69 (63%) | 2 (1%) | 20 (8%) |
| Dactylitis ever | 5 (4%) | 33 (30%) | 2 (1%) | 3 (1%) |
| Heel pain ever | 15 (12%) | 82 (75%) | 28 (14%) | 22 (9%) |
| Psoriasis ever | 13 (10%) | 48 (44%) | 15 (7%) | 9 (3%) |
| Uveitis ever | 14 (11%) | 10 (9%) | 20 (10%) | 13 (5%) |
| Inflammatory bowel disease ever | 3 (2%) | 8 (7%) | 12 (6%) | 24 (9%) |
| Inflammatory back pain ever | 97 (75%) | 104 (95%) | 132 (65%) | 157 (61%) |
| Elevated CRP (>5 mg/L) | 55 (42%) | 43 (39%) | 43 (21%) | 50 (19%) |
| HLA-B27 positive | 102 (78%) | 54 (49%) | 100 (49%) | 58 (22%) |
| Family history of SpA (ASAS) | 56 (43%) | 52 (47%) | 204 (100%) | 0 (0%) |
| Good response to NSAID ever | 59 (45%) | 57 (52%) | 65 (32%) | 69 (27%) |
Participants were individually categorised to one of the classes (obtained from the latent transition analysis) based on their posterior probability of class membership (with the class having the highest probability for each patient determining their assignment).
Mean (SD) or n (%).
Data for BMI available for 673 (96%) participants, for duration of back pain for 696 (99%) participants.
Missing values for imaging and clinical features of SpA have been imputed as described in the ‘Methods’ section.
ASAS, Assessment of SpondyloArthritis international Society; BME, bone marrow oedema; BMI, body mass index; CRP, C reactive protein; HLA-B27, human leucocyte antigen B27; 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.