The monitoring of Vitamin D deficiency during Chronic Kidney Disease (CKD) and Dialysis or End Stage Renal Disease (ESRD) is very important and has become a general practice in many countries. Total 25OH Vitamin D is typically measured annually in patients with CKD stages 3-4, or more frequently if the supplementation strategy does not increase the 25OH Vitamin D concentrations as expected.

As kidney function worsens, low circulating Vitamin D metabolitescan lead to secondary hyperparathyroidism (SHPT). In addition Vitamin D may play a role in complications of CKD, including hypertension, left ventricular hypertrophy and diastolic dysfunction, albuminuria, vascular calcification and infection.

Due to a decrease in the concentration of the Vitamin D Binding Protein during nephrotic syndrome the measurement of total 25OH Vitamin D is no longer a good representation of the concentration of the free 25OH Vitamin D.

The total 25OH Vitamin D test underestimates the free fraction, with the risk of Vitamin D status misclassification.

Recent research has shown that the measurement of free 25OH Vitamin D during renal disease seems to be a better marker of Vitamin D deficiency than the current total 25OH Vitamin D test. Additional clinical studies are currently initiated in order to bring more evidences.