The reference method for vitamin D analysis has been liquid chromatography - mass spectrophotometry (LC-MS/MS), which can measure vitamin D2, vitamin D3, and the D3 epimer (one of a pair of stereoisomers) separately and, through a calculation, the total vitamin D is reported. This method was chosen by the Nutritional Laboratory at the Centers for Disease Control and Prevention (CDC) and the National Laboratory in the United Kingdom for analysis of vitamin D for health and nutrition surveys, partly due to its ability to distinguish the various forms of vitamin D in plasma that may be found in people of all ages. The problem with these methods is that they are time and labor intensive and technically difficult to perform.
