Table 2

Baseline characteristics comparing patients with SpA and no-SpA according to the rheumatologist's follow-up diagnosis in each study population

All
(N=564)
Axial population
(N=394)
Peripheral population
(N=170)
SpA
(N=396)
No-SpA
(N=168)
axSpA
(N=280)
No-SpA
(N=114)
pSpA
(N=116)
No-SpA
(N=54)
Age (years) at baseline, mean (SD)31.2 (11.1)36.7 (10.5)30.7 (10.9)36.1 (10.8)32.1 (11.6)37.9 (9.6)
Age (years) at onset of back pain,* mean25.0 (8.5)27.7 (9.3)25.0 (8.5)27.7 (9.3)NANA
Onset of back pain before 40 years,* n265 (94.6)102 (89.5)265 (94.6)102 (89.5)NANA
Duration of back pain in years,* mean (SD)5.7 (7.2)7.9 (10.5)5.7 (7.2)7.9 (10.5)NANA
Male gender, n (%)224 (56.6)64 (38.1)147 (52.5)39 (34.2)77 (66.4)25 (46.3)
Number of SpA features,† mean (SD)2.9 (1.3)1.5 (1.1)3.1 (1.4)1.3 (1.1)2.5 (1.0)1.8 (0.8)
Presence of 2 or more SpA features,† n (%)339 (85.6)78 (46.4)241 (86.1)46 (40.4)98 (84.5)32 (59.3)
Definite radiographic sacroiliitis,*‡ n (%)70 (25.0)
(N=280)
4 (3.5)
(N=114)
70 (25.0)
(N=280)
4 (3.5)
(N=114)
NANA
Active inflammation of sacroiliac joints, MRI,* n (%)141 (63.8)
(N=221)
10 (11.2)
(N=89)
141 (63.8)
(N=221)
10 (11.2)
(N=89)
NANA
HLA-B27, n (%)247 (63.5)
(N=389)
28 (17.7)
(N=158)
191 (70.0)
(N=273)
26 (23.4)
(N=111)
56 (48.3)
(N=116)
2 (4.3)
(N=47)
Elevated CRP, n (%)192 (48.5)37 (22.2)123 (43.9)14 (12.3)69 (59.5)23 (42.6)
IBP (according to experts definition*§), n (%)224 (80.0)43 (37.7)224 (80.0)43 (37.7)NANA
Peripheral arthritis past or present, n (%)231 (58.3)68 (40.5)125 (44.6)19 (16.7)106 (91.4)49 (90.7)
Enthesitis past or present, n (%)206 (52.0)40 (23.8)136 (48.6)26 (22.8)70 (60.3)14 (25.9)
Uveitis past or present, n (%)34 (8.6)5 (3.0)30 (10.7)5 (4.4)4 (3.5)0 (0.0)
Dactylitis past or present, n (%)47 (11.9)8 (4.8)21 (7.5)1 (0.9)26 (22.4)7 (13.0)
Psoriasis past or present, n (%)29 (7.3)7 (4.2)17 (6.1)6 (5.3)12 (10.3)1 (1.9)
IBD past or present, n (%)16 (4.0)0 (0.0)11 (3.9)0 (0.0)5 (4.3)0 (0.0)
Active inflammation of the spine, MRI,* n (%)32 (34.8)
(N=92)
3 (5.0)
(N=60)
32 (34.8)
(N=92)
3 (5.0)
(N=60)
NANA
  • *Only applicable in patients from the axial population.

  • †Features included: IBP according to experts definition, arthritis (ever), heel enthesitis (ever), dactylitis (ever), uveitis (ever), psoriasis (ever), IBD (ever), good response to NSAIDs, family history of spondyloarthritis (SpA), elevated CRP.

  • ‡≥Grade 2 bilateral or ≥grade 3 unilateral. §IBP experts (four or more out of five): (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).

  • axSpA, axial spondyloarthritis; CRP, C reactive protein; IBD, inflammatory bowel disease; IBP, inflammatory back pain; NA, not applicable; pSpA, peripheral spondyloarthritis.

  • Although imaging of the axial skeleton was performed in some patients from the ‘peripheral population’, the significant amount of missing data precludes unbiased proportions to be calculated.