Vitamin D: many questions remain unanswered

Abstract

Vitamin D deficiency/insufficiency not only results in disorders of skeletal system, but may also be associated with other nonskeletal diseases. Therefore, measurement of 25-hydroxyvitamin D has received more and more attentions. Common methods for 25-hydroxyvitamin D testing include vitamin D binding protein-based competitive assays, immunoassays, high performance liquid chromatography (HPLC), and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Vitamin D binding protein-based assays and immunoassays can be performed on automated chemistry analyzers, permitting an easy and quick measurement with high throughput. However, significant variability exists among various assays. They cannot separate 25-hydroxyvitamin D3 and D2 either. HPLC method can separate D3 and D2, but the sensitivity and specificity are not as good as mass spectrometry. LC-MS/MS is more specific and sensitive and it can measure D3 and D2 individually. However, the methodology is more complicated and its throughput is relatively low. Also, it requires special instrument and operators who need additional training. In addition, most LC-MS/MS methods are developed and validated by individual laboratories, so the standardization of the method is an urgent problem to be solved. Although the specificity of LC-MS/MS method is high, some metabolites may still interfere with the assay. The reference interval of 25-hydroxyvitamin D is still controversial and how to establish gender- and age-specific reference intervals remains debatable. Besides 25-hydroxyvitamin D, whether its metabolites and free 25-hydroxyvitamin D should be measured is still questionable. Although 25-hydroxyvitamin D testing is primarily used for the diagnosis and treatment of skeletal diseases, epidemiological studies suggest that vitamin D deficiency/insufficiency may be related to the development and prognosis of nonskeletal diseases. However, more evidence is needed in order to determine the causal relationship between them and the mechanism. (Chin J Lab Med, 2017, 40: 744-747) Key words: Vitamin D; Chromatography, liquid; Tandem mass sectrometry