Clinical study of serum vitamin D binding protein in patients with rheumatoid arthritis associated with secondary osteoporosis

Abstract

Objective To explore the changes of serum vitamin D binding protein (VDBP) levels in patients with rheumatoid arthritis (RA) and the clinical significance of association between serum VDBP with secondary osteoporosis (OP) in RA. Methods One hundred and sixty patients with RA were enrolled in the study. Eighty-three normal subjects were recruited as the control group. The concentration of serum VDBP was determined by enzyme-linked immuno sorbent assay (ELISA), and bone mineral density (BMD) was measured by dual energy X-ray absorptiometry. Clinical and laboratory indexes of RA patients were recorded in detail and disease activity (DAS28) score, health assessment questionnaire (HAQ) and Sharp score according to X-ray examination of both hands were calculated simultaneously. The t test was used to compare the metrological data between the two groups, and the χ2 test was used to compare the intergroup rate. The correlation analysis was tested by Pearson correlation analysis, the ROC curve was used to analyze the threshold of the serum VDBP, and the multivariate logistic regression analysis was used for multivariate analysis. Results ① Serum levels of VDBP in RA patients were higher than that in control group [(414±12) ng/ml vs (79±12) ng/ml, t=20.082, P<0.01]. Positive rate of serum levels of VDBP was 67.0%(118/176) in RA patients, which was higher than that in the control group (4.8%) (χ2=87.651, P<0.01). ② The threshold of serum VDBP levels for diagnosing RA was 193.74 ng/ml (AUC=0.943, Youden index=0.796, P<0.01). ③ DAS28 sore in group with positive VDBP was significantly higher than that in group with negative VDBP (P=0.025). There was significant difference regarding on the incidence of OP in female RA patients between groups with positive and negative VDBP [41.9%(54/129) vs 31.8%(14/44), χ2=4.325, P=0.038]. ④ Linear correlation analysis found that DAS28 in RA patients was positively correlated with serum VDBP levels (r=0.252, P=0.019). And anti-CCP was negatively correlated with serum VDBP levels (r=-0.150, P=0.049). ⑤ Results of logistic regression analysis showed that sex [OR=9.841, 95%CI (1.349, 71.810), P=0.024], age [OR=1.154, 95 %CI (1.069, 1.245), P<0.01] and Sharp score [OR=1.102, 95%CI (1.002, 1.021), P=0.018] were risk factors for OP in RA patients. Conclusion Serum VDBP levels are significantly higher in patients with RA. Meanwhile, serum VDBP levels are correlated with disease activity and secondary OP in RA. Key words: Arthritis, rheumatoid; Osteoporosis; Vitamin D-binding protein