DBP and systemic lupus erythematosus: Serum 25(OH)D (ng/mL), D binding protein (DBP) (ng/mL), free (pg/mL) and bioavailable (ng/mL) vitamin D in Systemic Lupus Erythematosus patients, according to disease activity measured by Systemic Lupus Erythematosus Disease Activity Index (SLEDAI).