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Vitamin D determination

Determining vitamin D status in patients has become a serious clinical problem with a large proportion of the population now being defined as deficient or inadequate to maintain bone density. Vitamin D determination may appear as a simple analyte, which is, however, not necessarily true. In plasma the hydrophobic nature of the vitamin and its metabolites means that it has to be transported to a binding protein. Further complexity is added by (1) the different molecular forms of vitamin D and it’s metabolites present in serum (between 33 and 50 different compounds), (2) the short half-life of the parent vitamin itself, and (3) the selection of the metabolite to be measured that would adequately reflect the vitamin D status of the patient.

In western societies traditional reference ranges for serum 25(OH)D vitamin D are typically very wide and depending on the population under investigation can range from 10 to over 100 nmol/L. So far recommendations for 25(OH)D cut-off values range from as low as 30, 37.5, 50, 75, to as high as 110 nmol/L (4;5). The two most commonly used cut-off values for 25(OH)D are 50 and 75 nmol/L. A cut-off of 50 nmol/L is primarily promoted by the Institute of Medicine (IOM) in the United States of America. In 2010 the IOM revised the recommended dietary allowances (RDA) for vitamin D, to maintain serum 25(OH)D at or above 50 nmol/L to sustain bone density, calcium absorption, and to minimize risk of osteomalacia and rickets. Today most experts agree that there is compelling evidence for additional health benefits if 25(OH)D exceeds 75 nmol/L (4). These include fewer falls and fractures, less colorectal and breast cancer, cardiovascular disease, diabetes mellitus, and improved blood pressure.


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