US labs generally report 25(OH)D levels as ng/mL. Other countries use nmol/L. Multiply ng/mL by 2.5 to convert to nmol/L.This test can be used to diagnose vitamin D deficiency, and is performed in people with high risk for vitamin D deficiency, when the results of the test can be used to support beginning replacement therapy with vitamin D supplements. Patients with osteoporosis, chronic kidney disease, malabsorption, obesity, and some other infections may be at greater risk for being vitamin D-deficient and so are more likely to have this test. Although vitamin D deficiency is common in some populations including those living at higher latitudes or with limited sun exposure, the 25(OH)D test is not usually requested for the entire population. Physicians may advise low risk patients to take over-the-counter vitamin D supplements in place of having screening.It is the most sensitive measure, though experts have called for improved standardization and reproducibility across different laboratories. According to MedlinePlus, the recommended range of 25(OH)D is 20 to 40 ng/mL (50 to 100 nmol/L) though they recognize many experts recommend 30 to 50 ng/mL (75 to 125 nmol/L). The normal range varies widely depending on several factors, including age and geographic location. A broad reference range of 20 to 150 nmol/L (8-60 ng/mL) has also been suggested, while other studies have defined levels below 80 nmol/L (32 ng/mL) as indicative of vitamin D deficiency.Increasing calcifediol levels up to levels of 80 nmol/L (32 ng/mL) are associated with increasing the fraction of calcium that is absorbed from the gut.
Calcifediol has an elimination half-life of around 15 to 30 days.Calcifediol is further hydroxylated at the 1-alpha-position in the kidneys to form 1,25-(OH)2D3, calcitriol. This enzymatic 25(OH)D-1α-hydroxylase reaction is performed exclusively by CYP27B1, which is highly expressed in the kidneys where it is principally regulated by parathyroid hormone, but also by FGF23 and calcitriol itself. CYP27B1 is also expressed in a number of other tissues, including macrophages, monocytes, keratinocytes, placenta and parathyroid gland and extra-renal synthesis of calcitriol from calcifediol has been shown to have biological effects in these tissues.Calcifediol is also converted into 24,25-dihydroxycholecalciferol by 24-hydroxylation. This enzymatic reaction is performed by CYP24A1 which is expressed in many vitamin D target tissues including kidney, and is induced by calcitriol. This will inactivate calcitriol to calcitroic acid, but 24,25-(OH)2D3 may have some biological actions itself. Blood test for vitamin D deficiency
In medical practice, a  blood test for 25-hydroxy-vitamin D, 25(OH)D, is used to determine an individuals vitamin D status. The name 25(OH)D refers to any combination of calcifediol (25-hydroxy-cholecalciferol), derived from vitamin D3, and ercalcidiol (25-hydroxy-ergocalciferol), derived from vitamin D2. The first of these (also known as 25-hydroxy vitamin D3) is made by the body, or is sourced from certain animal foods or cholecalciferol supplements. The second (25-hydroxy vitamin D2) is from certain vegetable foods or ergocalciferol supplements. Clinical tests for 25(OH)D often measure the total level of both of these two compounds together, generally without differentiating.This measurement is considered the best indicator of overall vitamin D status.