Page 80 - Recognizing axial spondyloarthritis - Janneke de Winter

CHAPTER FIVE
We included 138 patients with AS and 57 healthy controls. The mean (SD) age was 46.1 (13.1) in patients with AS and 36.7 (13.5) in healthy controls (p<.001). Of the AS patients, 92 out of 117 were male vs. 22 out of 38 healthy controls (78.6% vs. 57.9%, p=0.012). HLA-B27 was determined in 117 out of 138 AS patients; 108 out of 117 patients were HLA-B27 positive (92.3%). The mean disease duration was 24.3 (13.2) months.
The SPACE cohort
We used baseline serum samples of the SPondyloArthritis Caught Early (SPACE) cohort. The SPACE cohort is an ongoing, prospective, multicenter, longitudinal cohort that started in 2009 and is described in detail earlier (11). In short, the SPACE cohort is including patients aged ≥16 years with chronic back pain (≥3 months, but ≤2 years) with the onset before the age of 45 years. The local medical ethical committees of the participating sites approved the study and all participants gave their written informed consent. For the present analyses, data and sera of the baseline visit were used. We used the central reading results of X rays and MRI of the sacroiliac joint (11). The primary outcome was an axSpA diagnosis by a rheumatologist vs. no axSpA diagnosis by a rheumatologist. As secondary outcomes, we used 1) a strict definition of axSpA: patients diagnosed by a rheumatologist as axSpA plus fulfilling the ASAS axSpA criteria vs. patients not diagnosed by a rheumatologist as axSpA and not fulfilling the ASAS axSpA criteria, and 2) SPACE patients diagnosed as axSpA by a rheumatologist at baseline and after one year of follow-up vs. patients twice not diagnosed as axSpA. Serum samples were collected at baseline and stored for at least 6 months by -80°C.
We included 593 patients of the SPACE cohort, including 274 patients diagnosed with axSpA at baseline. Table 1 shows baseline characteristics of the study patients. In short, 119 of the axSpA patients were male vs. 81 of the CBP patients (43.4% vs. 28.3%, p<0.001). The mean age at inclusion of the axSpA patients was 30.7 (7.9) and 31.4 (8.5) of the CBP patients (p=0.83). Of the axSpA patients, 165 were HLA-B27 positive versus 63 of the CBP patients (60.7% vs. 22.1%, p<0.001). Of the axSpA patients, 172 fulfilled the ASAS axSpA criteria and 48 of the CBP patients fulfilled the ASAS axSpA criteria (62.8% vs. 37.2%, p<0.001).
Missing values were below 5% except for axSpA diagnosis (5.6%), Swollen Joint Count (SJC, 5.7%), CRP (7.1%), sacroiliitis according to the modified New York (mNY) criteria (17.3%), Bath Ankylosing Spondylitis Disease Activity Index (BASDAI, 15.0%), total serum IgA (9.2%) and the number of patients missing the one-year evaluation (58.9%). This was mostly due to the study protocol: not all patients were invited for follow-up and some patients did not have a one-year follow-up visit due to a too short time of follow-up.
Anti-CD74 antibody detection
Two different enzyme-linked immunosorbent assays (ELISAs) were performed to detect anti-CD74 IgG and anti-CD74 IgA antibodies. The medical university
78