**2. Definition: vitamin D deficiency, vitamin D sufficiency, chronic kidney disease**

VD status is being evaluated via the serum level of 25-hydroxyvitamin D (25VD)—the metabolite formed in the first hydroxylation in the liver, due to its longer half-life (approx. 3 weeks), compared to the active metabolite 1,25-dihydroxyvitamin D (1,25VD) (4–6 hours). The best options for 25VD serum level evaluation methods are high performance liquid chromatography (HPLC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). There are reports, discussing the use of free 25VD as indicator of VD status. However, in renal disease,

the ratio between free and total 25VD remained unchained; therefore, total 25VD is the indicator of VD status in CKD [3].

Generally, 25VD level has reverse association with parathyroid hormone (PTH) levels. In addition, higher 25VD is associated with higher calcium intestinal reabsorption. However, at 25VD ≥ 75 nmol/l, no reduction in PTH levels and no increase in calcium intestinal reabsorption occurs [4, 5]. Therefore, 25VD ≥ 75 nmol/l is regarded as a cut-off value for vitamin D sufficiency.

No generally accepted definition for VD deficiency exists, as authors choose cut-off value either serum 25VD of 25 nmol/L [6] or serum 25VD of 50 nmol/L [7]. However, target levels of 75 nmol/L are recommended, taking into consideration the clinical importance of mild VD insufficiency (25VD between 50 and 74.9 nmol/L) [7, 8].

Chronic kidney disease (CKD) is defined as abnormalities of kidney structure or function, present for more than 3 months, with implications for health [one of the following: estimated glomerular filtration rate (eGFR) less than 60 ml/ min/1.73 m<sup>2</sup> ,presence of proteinuria, structural abnormalities of the kidney detected on imaging, pathological findings detected on kidney biopsy, urine sediment pathology, electrolyte abnormalities due to tubular disorders, history of kidney transplantation] [9]. CKD patients are at increased risk for VD deficiency; therefore, they require VD screening [7].
