Standardized measurement of circulating vitamin D [25(OH)D] and its putative role as a serum biomarker in Alzheimer's disease and Parkinson's disease.

The current review provides an overview on the development of 25(OH)D measurement standardization tools over the last three decades and clarifies whether there is a role as a serum biomarker for vitamin D in neurological diseases. In the past, a lack of internationally recognized 25(OH)D reference measurement procedures and reference standard materials led to unstandardized serum total 25(OH)D results among research and clinical care laboratories. The vitamin D Standardization Program (VDSP) has been introduced in 2010 to address this problem, however, vitamin D External Quality Assessment Scheme (DEQAS) reports still show substantial sample- to- sample variability. Further, immunoassays, which are mainly used in clinical care laboratories, display analytical issues, including matrix-effects interferences, which cannot be overcome by the standardization process. Hence, liquid chromatography-tandem mass spectrometry (LC/MS-MS) methods should be used to measure 25(OH)D. Low vitamin D serum levels have been found in patients affected by Alzheimer's disease and Parkinson's disease, suggesting a role for vitamin D as a serum biomarker in these diseases. However, few studies reported 25(OH)D standardized results, thus, no clear evidence on the potential role of 25(OH)D serum levels in these diseases exists.