Page 85 - Recognizing axial spondyloarthritis - Janneke de Winter

Anti-CD74 IgG (OD)
Anti-CD74 IgA (U/ mL)
Total IgA (g/L)
0.50 0.52 0.15 (0.32- (0.38-
0.65) 0.69)
19.9 14.0 <0.001 (11.4- (9.3-
30.6) 26.0)
4.07 3.37 <0.001 (2.90- (2.57-
4.97) 4.66)
0.50 0.52 0.075 (0.30- (0.39-
0.67) 0.70)
21.8 13.5 <0.001 (11.6- (8.4-
31.9) 22.3)
4.06 3.29 0.001 (3.01- (2.42-
5.03) 4.62)
P-value
Yes No (n=172) (n=238)
P-value
ANTI-CD74 ANTIBODIES IN EARLY AXIAL SPA
Total serum IgA
Since earlier studies showed that total serum IgA is elevated in AS patients (12,13), we additionally measured the level of total serum IgA in the SPACE cohort. ROC curve analysis showed an AUC of 0.59. Total IgA levels were higher in axSpA patients than in CBP patients (4.07 vs. 3.37 g/L, p<0.001, Table 2). Total serum IgA was higher than the upper reference range (4.0 g/l) in 49.0% of the axSpA and in 35.8% of the CBP patients (p=.008). Anti-CD74 IgA and total IgA were both statistically significant predictors for a diagnosis of axSpA in a univariate logistic regression model (data not shown). We tested for collinearity by first measuring Pearson’s correlation coefficient, which was 0.31 for total IgA and anti-CD74 IgA (p<0.001). Second, we determined the VIF in a linear regression model, showing that the amount of collinearity was low (a VIF of 1.11 and R2 of 0.90). In a multivariate logistic regression model including anti-CD74 IgA and total IgA, total serum IgA was associated with a diagnosis of axSpA (OR=1.19, p<0.001) whereas anti-CD74 IgA was not (OR=1.01, p=0.33). Adding other potential confounding variables to the model (CRP level, inflammatory back pain, HLA-B27, inflammatory bowel disease) did not change the results.
Table 2. Anti-CD74 antibody and IgA levels in sera of patients of the SPACE cohort.
AxSpA (diagnosis)
Yes No (n=274) (n=286)
Diagnosis and ASAS axSpA criteria
AxSpA (diagnosis) confirmed at 1 year follow-up
Yes No P-value (n=143) (n=64)
0.43 0.48 0.32 (0.30- (0.35-
0.58) 0.61)
17.6 13.7 0.10 (9.9- (9.4-
29.0) 23.6)
4.02 3.66 0.14 (2.77- (2.43-
4.78) 4.74)
Results are shown as median (IQR). Axial SpA (diagnosis): SPACE participants diagnosed with axSpA by a rheumatologist at baseline; ASAS axSpA criteria: SPACE participants fulfilling vs. not fulfilling the ASAS axSpA criteria at baseline; Diagnosis and ASAS axSpA: SPACE participants diagnosed as axSpA and fulfilling the ASAS axSpA criteria vs. patients not diagnosed as axSpA and not fulfilling the ASAS axSpA criteria. ASAS=Assessment of SpondyloArthritis international Society; axSpA=axial spondyloarthritis. Missing values were 5.6% for ‘diagnosis of axSpA’, 3% for ‘fulfilling the ASAS axSpA criteria’, 30.9% for ‘diagnosis and ASAS axSpA criteria’ and 58.9% for ‘axSpA diagnosis at 1 year follow-up’.
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