The D‐vitamin metabolite 1,25( OH ) 2 D in serum is associated with disease activity and Anti‐Citrullinated Protein Antibodies in active and treatment naïve, early Rheumatoid Arthritis Patients

Rationale Sufficient levels of vitamin D seem to be essential for proper immune function, and low levels might be associated to disease activity in Rheumatoid Arthritis ( RA ). Most studies investigate only 25 OHD and not the physiologically active vitamin D metabolite, 1,25( OH ) 2 D. Objective To investigate associations between serum level of vitamin D metabolites and disease activity parameters in 160 inflammatory active and treatment naïve early RA patients. Serum level of vitamin D metabolites (25 OHD 2 , 25 OHD 3 and 1,25( OH ) 2 D) was measured by isotope dilution mass spectrometry and radio‐immunoassays at baseline. Disease characteristics were gender, number of tender joints, number of swollen joints, DAS 28‐ CRP , HAQ , VAS ‐scores, CRP , erosive status (Total Sharp Score; TSS ), ACPA and IgM‐ RF ‐status. Associations were evaluated using Spearman's and Wilcoxon rank‐sum tests. The study was registered in clinical trials; trial registration number: NCT 00209859. Findings Statistically significant inverse associations were found between the active metabolite 1,25( OH ) 2 D and DAS 28‐ CRP ( P  = 0.004, rho = −0.23), HAQ ( P  = 0.005, rho = −0.22), CRP ( P  = 0.001, rho = −0.25), VAS patient‐pain ( P  = 0.008, rho = −0.21), and a positive association was found to ACPA ‐status ( P  = 0.04). Conclusion The vitamin D metabolite 1,25( OH ) 2 D was inversely associated with disease activity and positively associated with ACPA in treatment naïve and inflammatory active early RA . The results indicate that in RA , both the degree of inflammatory activity, and the diagnostic sensitivity and specificity might affect—or might be affected by the level of vitamin 1,25( OH ) 2 D.