Purpose Systemic lupus erythematosus (SLE) is a multisystem autoimmune disorder which mainly affects women in the reproductive age. Vitamin D appears to have an immunomodulatory action, low levels of vitamin D having been described in patients with autoimmune rheumatic diseases. The aim was to report vitamin D levels and their relationship with disease activity in a cohort of SLE patients.

Methods A cohort of 20 patients, 18 female and 2 male is described. The patients were diagnosed with SLE and had arthralgias, cutaneous manifestations, fatigue and decreased complement levels. Blood levels of 25(OH)D3 were measured in all patients. 25(OH)D3 levels were measured by radioimmunoassay. The measurement of 25(OH)D3 by radioimmunoassay consisted of a two-step procedure. The first step involved a rapid extraction of 25(OH)D and other hydroxylated metabolites from serum or plasma with acetonitrile. Following extraction, the treated samples were then assayed by competitive RIA using an antibody with specificity to 25OHD. The sample, antibody and tracer were incubated for 90 min at 20–25 0C. Phase separation was accomplished after 20 min incubation at 20–25 0C with a second antibody precipitating complex. To reduce non-specific binding buffer was added after this incubation prior to centrifugation. The sensitivity of the assay was <1.6 ng/ml. The recovery was approximately 100% for 25(OH)D3. Within and between batch precision was <12% and<11%, respectively.

Results In the cohort of SLE patients low blood levels of 25(OH)D3 were observed. A positive relationship between 25(OH)D3 blood levels and complement levels was observed, namely low 25(OH)D3 levels were positively correlated with low complement levels. An inverse relationship was observed between 25(OH)D3 levels and disease activity, namely low 25(OH)D3 levels were related with high disease activity.

Conclusions Vitamin D is a hormone directly related to the regulation of the musculoskeletal system. Vitamin D also has extraskeletal actions. The immunomodulatory action of vitamin D appears to be a key action of the hormone. In the work presented herein low blood levels of vitamin D were observed in SLE patients which were positively related to complement levels and inversely related to disease activity.