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Because vitamin D deficiency has been previously reported in patients with systemic lupus erythematosus (SLE), we decided to examine the prevalence of vitamin D deficiency in Thai SLE patients, to identify possible independent factors affecting serum 25-hydroxyvitamin D2 and D3 [25(OH)] vitamin D levels, and to examine the associations of serum 25(OH) vitamin D and disease activity and damage in Thai SLE patients.A cross-sectional study was performed in 101 SLE patients. Blood samples were prospectively collected. The levels of 25(OH) vitamin D were measured by radioimmunoassay. The cutoffs for vitamin D deficiency and insufficiency were 30 and 20 ng/mL, respectively. Demographic, clinical, and laboratory data were collected, and their associations with 25(OH) vitamin D level were examined by univariate and multivariate linear regression analyses.The level of 25(OH) vitamin D (mean [SD]) was 27.9 (7.6). Seventeen patients (17%) had vitamin D deficiency, 41 patients (41%) had vitamin D insufficiency, and 43 patients (42%) had normal vitamin D levels. Two thirds of the patients were taking relatively low-dose vitamin D supplementations. Current daily glucocorticoid dose and serum creatinine levels were negatively correlated with vitamin D levels (β = −0.207, P = 0.023; and β = −3.770, P = 0.003, respectively). There were no associations between disease activity or damage and 25(OH) vitamin D levels.Vitamin D deficiency and insufficiency are common in SLE patients despite more than half of them taking vitamin D supplementations. Higher serum creatinine level and higher current daily glucocorticoid dose are associated with lower serum 25(OH) vitamin D levels. These patients may require higher doses of vitamin D supplementations.