Vitamin D deficiency is currently evaluated by the measurement of Total 25OH Vitamin D. Various methods exist, including physical detection methods such as HPLC and LC-MS, and immuoassays such as CLIA, RIA  and ELISA.

It is used since three decades in the diagnostic and monitoring of Vitamin D deficiency linked diseases such as rickets, osteoporosis and osteomalacia. Although bone-related issues remain the main field of interest for the measurement of Vitamin D metabolites, the massive amount of research that has been performed during the last decade has led to a number of novel clinical applications, with a large portion still being to be fully proven by large random clinical trials. This continuously increasing interest for the action of Vitamin D and for the implications of its deficit has led over time to a real explosion of the number of tests performed by the laboratories.

However, and despite the huge number of studies that have been performed since more than 30 years, no clear consensus on the recommended 25OH Vitamin D concentrations has been established.

Different reasons can explain such a situation. The fact that Vitamin D has a positive action in the prevention of many different diseases is certainly one of the reason why it is difficult to establish clear and simple guidelines.

The second reason is linked to the marker that has been used so far to evaluate Vitamin D deficiency: total 25OH Vitamin D. Recent publications are showing that measuring free 25OH Vitamin D might be a better indicator of Vitamin D action in several conditions such as pregnancy, renal disease, liver disease and intensive care.