Clinical diagnostic tools for vitamin D assessment

Vitamin D deficiency has been implicated in a plethora of diseases including rheumatoid arthritis, Parkinson's disease, Alzheimer's disease, and osteoporosis. Deficiency of this vitamin is a global epidemic affecting both developing and developed nations. Within a clinical context, the qualitative and quantitative analysis of vitamin D is therefore vital. The main metabolic markers for assessing vitamin D status in humans are the hydroxylated forms of vitamin D, 25OHD ₃ and 25OHD ₂ on account of their long half-lives within the body and excellent stability. An adequate level for healthy individuals of these hydroxylated forms is estimated to be around 20–40 ng/ml of blood. There are three main analytical techniques for determining the levels of 25OHD ₃ and 25OHD ₂ . The first technique is immunoassay-based and can be performed in a rapid, high throughput, automated manner, allowing as many as 240 tests per hour with the duration of each assay as little as 18 min. Furthermore, it offers excellent sensitivity with a detection range of 3.4–156 ng/ml. A major downside of immunoassays is that they are unable to distinguish between the various forms of vitamin D. While HPLC is a highthroughput low cost instrument it is not a very sensitive technique and cannot quantify the down stream metabolites of vitamin D. The third technique, namely liquid chromatography-mass spectrometry (LC–MS/), provides excellent sensitivity with a wide dynamic range from 0.068 pg/ml to 100 ng/ml. Additionally, it offers a high level of separation and permits identification of vitamin D-related metabolites. However, a huge limitation with LC/MS/MS is their poor throughput for sample analyses. As yet, there is no analytical technique which combines the fine detection capabilities of LC/MS/MS and the rapid, automated format of immunoassay, for vitamin D analyses. Future attention therefore needs to be given to this area if the current clinical diagnostic tools for vitamin D analysis are to be further improved.