| Clinical criterion | Definition |
| IBP | IBP according to experts (see also Box 5): four out of five of the following 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 doctor |
| Family history | Presence in first-degree or second-degree relatives of any of the following: (a) ankylosing spondylitis, (b) psoriasis, (c) uveitis, (d) reactive arthritis, (e) inflammatory bowel disease |
| Psoriasis | Past or present psoriasis diagnosed by a doctor |
| Inflammatory bowel disease | Past or present Crohn disease or ulcerative colitis diagnosed by a doctor |
| Dactylitis | Past or present dactylitis diagnosed by a doctor |
| Enthesitis | Heel enthesitis: past or present spontaneous pain or tenderness at examination at the site of the insertion of the Achilles tendon or plantar fascia at the calcaneus |
| Uveitis anterior | Past or present uveitis anterior, confirmed by an ophthalmologist |
| Good response to NSAIDs | At 24–48 h after a full dose of NSAID the back pain is not present anymore or much better |
| HLA-B27 | Positive testing according to standard laboratory techniques |
| Elevated CRP | CRP above upper normal limit in the presence of back pain, after exclusion of other causes for elevated CRP concentration |
| Sacroiliitis by x rays | Bilateral grade 2–4 or unilateral grade 3–4, according to the modified New York criteria (see Part C, Box 2) |
| Sacroiliitis by MRI | Active inflammatory lesions of sacroiliac joints with definite bone marrow oedema/osteitis suggestive of sacroiliitis associated with spondyloarthritis (see also Part B on MRI) |
CRP, C-reactive protein; HLA, human leukocyte antigen; IBP, inflammatory back pain; NSAID, non-steroidal anti-inflammatory drug.