Measurement and quality assessment of total and free 25-hydroxyvitamin D

This chapter is divided into two sections. An essential point in both is that serum total 25-hydroxyvitamin D (25(OH)D) is the clinically accepted biomarker for vitamin D status. Section 1: Section 1 will focus on standardizing serum total 25(OH)D measurements using the tools and methods developed by the Vitamin D Standardization Program (VDSP). In addition, special attention is given to the role that accuracy-based Performance Testing/External Quality Assessment (PT/EQA) schemes—especially the Vitamin D External Quality Assessment scheme (DEQAS)—can play in the continuous 25(OH)D standardization in all laboratories. Section 2: Since serum vitamin D binding protein and albumin bind 25(OH)D, only a small fraction of total 25(OH)D exists as free 25(OH)D and has been suggested as an alternative indicator of vitamin D status. Bioavailable 25(OH)D, free 25(OH)D plus 25(OH)D bound albumin, has been proposed as another metric. This section reviews the concepts and assays behind these alternatives.