| SpA feature | Definition |
| IBP | IBP according to experts:14 at least four out of five parameters present: (1) age at onset <40 years; (2) insidious onset; (3) improvement with exercise; (4) no improvement with rest; (5) pain at night (with improvement upon getting up) |
| Arthritis | Past or present active synovitis diagnosed by a physician |
| Enthesitis (heel) | Heel enthesitis: past or present spontaneous pain or tenderness at examination of the site of the insertion of the Achilles tendon or plantar fascia at the calcaneus |
| Uveitis | Past or present uveitis anterior, confirmed by an ophthalmologist |
| Dactylitis | Past or present dactylitis, diagnosed by a physician |
| Psoriasis | Past or present psoriasis, diagnosed by a physician |
| IBD | Past or present Crohn’s disease or ulcerative colitis diagnosed by a physician |
| Good response to NSAID | 24–48 h after a full dose of a NSAID the back pain is not present any more or is much better |
| Family history of SpA | Presence in first-degree (mother, father, sisters, brothers, children) or second-degree (maternal and paternal grandparents, aunts, uncles, nieces and nephews) relatives of any of the following: (1) AS; (2) psoriasis; (3) acute uveitis; (4) reactive arthritis; (5) IBD |
| Elevated CRP | CRP concentration above upper normal limit in the presence of back pain, after exclusion of other causes for elevated CRP concentration |
| HLA-B27 | Positive testing according to standard laboratory techniques |
| Sacroiliitis by radiographs | Bilateral grade 2–4 or unilateral grade 3–4 sacroiliitis on plain radiographs, according to the modified New York criteria3 |
| Sacroiliitis by MRI | Active inflammatory lesions of sacroiliac joints with definite bone marrow oedema/osteitis, suggestive of sacroiliitis associated with SpA22 |
AS, ankylosing spondylitis; CRP, C-reactive protein; IBD, inflammatory bowel disease; IBP, inflammatory back pain; MRI, magnetic resonance imaging; NSAID, non-steroidal anti-inflammatory drug; SpA, spondyloarthritis.