Vitamin D: marker, measurand & measurement

The measurement of vitamin D metabolites aids in assessing vitamin D status and in diagnosing disorders of calcium homeostasis. Most laboratories measure total 25-hydroxyvitamin D (25(OH)D), while others have taken the extra effort to measure 25(OH)D-2 and 25(OH)D-3 separately and additional metabolites such as 1,25-dihydroxyvitamin D and 24,25-dihydroxyvitamin D. The aim of this review is to provide an updated overview of the main markers of vitamin D metabolism, define the intended measurands, and discuss the advantages and disadvantages of the two most widely used assays, automated assays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Whether using the easy and fast automated assays or the more complex LC-MS/MS, one should know the pitfalls of the used technique in order to interpret the measurements. In conclusion, automated assays are unable to accurately measure 25(OH)D in all patient groups, including persons using D-2. In these cases, an LC-MS/MS method, when appropriately developed and standardized, produces a more reliable measurement.

Automated summary

Measurement of vitamin D metabolites is essential for assessing vitamin D status and diagnosing calcium homeostasis disorders. While most laboratories measure total 25-hydroxyvitamin D (25(OH)D), some also measure 25(OH)D-2, 25(OH)D-3, and additional metabolites. This review provides an overview of vitamin D metabolism markers, discusses the advantages and disadvantages of automated assays and LC-MS/MS, and emphasizes the need for understanding the limitations of each technique.