Free vitamin D levels in steroid-sensitive nephrotic syndrome and healthy controls

Introduction Body stores of vitamin D are measured as total serum 25-hydroxy vitamin D (25(OH)D). Its largest component is protein bound and lost in urine in nephrotic syndrome (NS). Our study investigates whether free 25(OH)D levels are a better guide to bone health and need for vitamin D supplementation in patients with steroid-sensitive NS (SSNS). Methods A cross-sectional study was performed in children with SSNS and healthy controls. Blood was tested for albumin, creatinine, calcium, phosphate, ALP, total and free (by direct ELISA) 25(OH)D, iPTH, and urine for protein-creatinine ratio. Results Seventy-nine NS patients (48 in relapse, 31 in remission) and 60 healthy controls were included. The levels of total 25(OH)D were significantly different (lowest in NS relapse and highest in controls) (p < 0.001). Corrected calcium and phosphate levels were normal, and there were no differences in free 25(OH)D, ALP, or iPTH levels between groups. Only total and not free 25(OH)D correlated significantly and negatively with urinary protein creatinine ratios (r(s) = - 0.42 vs. 0.04). Free 25(OH)D values of 3.75 and 2.85 pg/ml corresponded to total 25(OH)D levels of 20 and 12 ng/ml, respectively, in healthy controls. Conclusion These results confirm that total 25(OH)D levels are low in NS and related to degree of proteinuria. However levels of free 25(OH)D, ALP, and iPTH did not change in relapse or remission in comparison with healthy controls. Our results suggest that in proteinuric renal diseases, free 25(OH)D rather than total 25(OH)D levels should be used to diagnose vitamin D deficiency and guide therapy.