Page 78 - Recognizing axial spondyloarthritis - Janneke de Winter

CHAPTER FIVE
ABSTRACT
Background
Anti-CD74 IgG antibodies are reported to be elevated in axial spondyloarthritis (axSpA) patients. This study assessed the diagnostic value of anti-CD74 antibodies in early axSpA patients.
Methods
Anti-CD74 IgG and IgA antibodies were first measured in an exploratory cohort of patients with radiographic axSpA (ankylosing spondyloarthritis, (AS)) (n=138) and healthy controls (n=57) and second in patients with early axSpA (n=274) and non-SpA chronic back pain (CBP) patients (n=319) from the SPondyloArthritis Caught Early (SPACE) cohort, a prospective cohort study of patients under 45 years old with early back pain (≥3 months, but ≤2 years).
Results
In the exploratory cohort, anti-CD74 IgG antibodies were present in 79.7% of AS patients vs. 43.9% of healthy controls (p<0.001). Anti-CD74 IgA antibodies were present in 28.5% of AS patients vs. 5.3% of healthy controls (p<0.001).
In the SPACE cohort, anti-CD74 IgG antibody levels were present in 46.4% of the axSpA patients vs. 47.9% of the CBP patients (p=0.71). Anti-CD74 IgA antibodies were present in 54.7% of the axSpA patients and 37.0% of the CBP patients (p<0.001). This resulted in a positive predictive value of 58.8% (compared to a prior probability of 46.2%) and a negative predictive value of 59.1% (compared to a prior probability of 53.8%). In a regression model, total serum IgA was associated with axSpA (Exp(B) 1.19, p<0.001) whereas anti-CD74 IgA was not (Exp(B) 1.01, p=0.33). Furthermore, anti-CD74 IgA was associated with sacroiliitis on MRI (Exp(B)=2.50, p=.005) and heel enthesitis (Exp(B)=2.56, p=.002).
Conclusions
Albeit anti-CD74 IgA is elevated in early axSpA patients, this elevation is not sufficiently specific to yield significant diagnostic value in patients under 45 presenting with early back pain.
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