Table 1

Baseline characteristics comparing patients with and without follow-up data available and comparing patients assessed by telephone with those with follow-up visit

Follow-up data availabilityFollow-up assessment
Follow-up visit (N=345)Telephone assessment (N=219)
All
(N=909)
No data available
(N=345)
Data available
(N=564)
p Value*Axial
(N=246)
Peripheral
(N=99)
Axial
(N=148)
Peripheral
(N=71)
Age (years) at baseline, mean (SD)34.1 (11.5)36.1 (11.6)32.8 (11.2)<0.00133.2 (10.9)34.5 (10.5)30.9 (11.4)33.3 (12.4)
Age (years) at onset of back pain,† mean (SD)26.3 (9.1)27.0 (9.5)25.8 (8.8)0.08526.0 (8.8)NA25.4 (8.9)NA
Onset of back pain before 40 years,† n (%)597 (90.7)230 (87.1)367 (93.2)0.009230 (93.5)NA137 (92.6)NA
Duration of back pain in years,† mean (SD)7.4 (9.3)8.9 (10.3)6.3 (8.3)<0.0017.0 (8.9)NA5.2 (7.2)NA
Male gender, n (%)432 (47.6)144 (41.9)288 (51.1)0.007111 (45.1)55 (55.6)73 (49.3)24 (33.8)
SpA clinical diagnosis at baseline, n (%)574 (63.2)176 (51.0)398 (70.6)<0.001185 (75.2)72 (72.7)96 (64.9)45 (63.4)
ASAS criteria for SpA,‡ n (%)506 (55.7)171 (49.6)335 (59.4)0.004NANANANA
ASAS criteria for axSpA,† n (%)367 (55.8)127 (48.1)240 (60.9)0.001158 (64.2)NA82 (55.4)NA
ASAS criteria for pSpA,§ n (%)139 (55.4)44 (54.3)95 (55.9)0.816NA58 (58.6)NA37 (52.1)
Number of SpA features,¶ mean (SD)2.4 (1.4)2.2 (1.4)2.5 (1.4)0.0202.6 (1.5)2.3 (1.1)2.4 (1.6)2.2 (1.0)
Presence of 2 or more SpA features,¶ n (%)657 (72.3)240 (69.6)417 (73.9)0.153183 (74.4)74 (74.8)104 (70.3)56 (78.9)
Definite radiographic sacroiliitis,†** n (%)122 (18.6)
(N=657)
48 (18.2)
(N=264)
74 (18.8)
(N=392)
0.83453 (21.5)NA21 (14.3)NA
Active inflammation of sacroiliac joints, MRI,† n (%)208 (41.0)
(N=507)
57 (28.9)
(N=197)
151 (48.7)
(N=310)
<0.001101 (51.3)
(N=197)
NA50 (44.3)
(N=113)
NA
HLA-B27, n (%)407 (46.2)
(N=881)
132 (39.5)
(N=334)
275 (50.3)
(N=547)
0.002137 (56.2)
(N=244)
32 (33.3)
(N=96)
80 (57.1)
(N=140)
26 (38.8)
(N=67)
Elevated CRP, n (%)345 (38.0)116 (33.6)229 (40.6)0.03580 (32.5)51 (51.5)57 (38.5)41 (57.8)
IBP (according to experts definition),† n (%)415 (63.1)148 (56.1)267 (67.8)0.002175 (71.1)NA92 (62.2)NA
Peripheral arthritis past or present, n (%)454 (49.9)155 (44.9)299 (53.0)0.01895 (38.6)89 (89.9)49 (33.1)66 (93.0)
Enthesitis past or present, n (%)376 (41.4)130 (37.7)246 (43.6)0.078101 (41.1)49 (49.5)61 (41.2)35 (49.3)
Uveitis past or present, n (%)69 (7.6)30 (8.7)39 (6.9)0.32529 (11.8)3 (3.0)6 (4.1)1 (1.4)
Dactylitis past or present, n (%)81 (8.9)26 (7.5)55 (9.8)0.25515 (6.1)22 (22.2)7 (4.7)11 (15.5)
Psoriasis past or present, n (%)68 (7.5)32 (9.3)36 (6.4)0.10817 (6.9)10 (10.1)6 (4.1)3 (4.2)
IBD past or present, n (%)23 (2.5)7 (2.0)16 (2.8)0.4529 (3.7)3 (3.0)2 (1.4)2 (2.8)
Active inflammation of the spine, MRI,† n (%)46 (18.3)
(N=251)
11 (11.1)
(N=99)
35 (23.0)
(N=152)
0.01726 (26.3)
(N=99)
NA9 (17.0)
(N=53)
NA
  • 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.

  • 2 test for categorical variables and the independent samples t test for continuous variables.

  • †Only applicable in patients from the ‘axial population’ (N=658 at baseline and N=394 at follow-up).

  • ‡Combination of ASAS criteria for axSpA and criteria for pSpA.

  • §Only applicable in patients from the ‘peripheral population’ (N=251 at baseline and N=170 at follow-up).

  • ¶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.

  • ASAS, Assessment of SpondyloArthritis international Society; axSpA, axial spondyloarthritis; CRP, C reactive protein; IBD, inflammatory bowel disease; IBP, inflammatory back pain; NA, not applicable; pSpA, peripheral spondyloarthritis.