Total 25(OH) vitamin D: a new assay that delivers diagnostic confidence

The clinical importance of vitamin D, the so-called sunshine vitamin, has risen exponentially as links have been uncovered between the vitamin, or lack of it, and poor health. Vitamin D deficiency has been implicated in conditions such as osteoporosis, autoimmune disorders, heart disease, tuberculosis and even certain cancers. This has increased pressure on GPs and laboratory services for more routine testing. In many cases, it has not yet been established if the deficiency is actually a symptom of the disease or a contributing factor. However, it all places an extra burden on NHS resources.

Currently, there is no international measurement standard for total 25-hydroxy vitamin D and this has led to a wide variation in the measurement results between different manufacturers’ tests. For many other immunoassays, manufacturers are able to use reference standards from the World Health Organization (WHO). Unfortunately, this is not yet available for vitamin D.

As test results accumulated, it was noted that vitamin D assays from different commercial sources and platforms produced inconsistent results from the same patient specimen. In some instances, these differences were large enough to affect whether a patient would be classified as having sufficient or deficient vitamin D levels. This was often because manufacturers’ assays used different methods to release vitamin D from its binding proteins, and the level of interference from heterophilic antibodies.