Table 3 Multivariable logistic regression of parameters contributory to the classification of axial SpA in patients with chronic back pain and age at onset 45 years or less
All patientsPatients who did not fulfil the imaging arm†
OR (95% CI)p ValueOR (95% CI)p Value
Male gender2.1 (1.1 to 4.1)0.0262.2 (1.2 to 3.9)0.007
IBP (according to experts; 4 of 5)*3.0 (1.6 to 5.9)0.0012.2 (1.2 to 3.8)0.006
Good response to NSAID2.7 (1.4 to 5.1)0.022.8 (1.6 to 4.9)<0.001
Enthesitis of the heel, current or past (%)0.7 (0.3 to 1.8)ns1.0 (0.5 to 2.1)ns
Peripheral arthritis, current or past (%)3.7 (1.7 to 8.0)0.0012.7 (1.5 to 5.0)0.002
Uveitis, current or past1.7 (0.6 to 4.7)ns1.4 (0.6 to 3.5)ns
Dactylitis, current or past5.3 (0.9 to 29.4)ns7.2 (1.5 to 33.3)0.012
Psoriasis, current or past2.5 (0.7 to 8.8)ns2.5 (0.9 to 7.2)ns
IBD, current or past6.5 (0.6 to 66.7)ns3.9 (0.5 to 33.3)ns
Family history of SpA (AS, reactive arthritis, uveitis, psoriasis, IBD)1.3 (0.6 to 2.8)ns1.5 (0.8 to 2.7)ns
Anterior lumbar flexion (Schober’s test; cm)0.9 (0.8 to 1.0)0.0460.8 (0.7 to 0.9)0.003
Lateral lumbar flexion (cm)0.9 (0.9 to 1.0)0.0040.9 (0.9 to 1.0)0.021
Chest expansion, cm1.1 (0.9 to 1.3)ns1.1 (0.9 to 1.3)ns
HLA-B272.8 (1.5 to 5.4)0.0022.7 (1.6 to 4.8)<0.001
Elevated CRP, above upper normal limit1.6 (0.7 to 3.3)ns1.9 (1.0 to 3.6)ns
Definite radiographic sacroiliitis (⩾ grade 2 bilateral or ⩾ grade 3 unilateral)32.3 (6.6 to 166.7)<0.001NA
Unilateral grade 2 radiographic sacroiliitis7.8 (1.3 to 50)0.0288.2 (2.0 to 33.3)0.003
Active inflammation of sacroiliac joints, MRI (n  =  495)66.7 (22.7 to 200)<0.001NA
  • The variables definite radiographic sacroiliitis and active inflammation of sacroiliac joints on magnetic resonance imaging (MRI) were not entered into the logistic regression analysis of patients who did not fulfil the imaging arm. *Inflammatory back pain (IBP) according to experts; at least four of five parameters present.14 †n  =  374 patients who did not fulfil the imaging arm were analysed. The imaging arm of the criteria refers to the presence of definite radiographic sacroiliitis (grade 2 bilaterally or grade 3–4 unilaterally) or active sacroiliitis on MRI in conjunction with at least one clinical parameter. AS, ankylosing spondylitis; CRP, C-reactive protein; IBD, inflammatory bowel disease; NA, not assessed; ns, non-significant; NSAID, non-steroidal anti-inflammatory drug; SpA, spondyloarthritis.