It\'s estimated most U.S. adults may be deficient in vitamin D but blood sample testing may yield inaccurate measurements of vitamin D levels, researchers say. Karen Phinney of the National Institute of Standards and Technology, an agency of the U.S. Department of Commerce, and colleagues said medical research suggests vitamin D deficiency or insufficiency may be more common than previously thought. Measuring vitamin D itself doesn\'t work because it is rapidly changed into another form in the liver, Phinney said. \"That\'s why current methods detect levels of a vitamin D metabolite, or 25(OH)D. However, the test methods don\'t always agree and produce different results,\" Phinney said in a statement. To help laboratories detect consistent and accurate methods, the researchers developed a Standard Reference Material, or SRM 972, the first certified reference material for the determination of the metabolite in human serum -- a component of blood. The researchers developed four versions of the standard, with different levels of the vitamin D metabolites 25(OH)D2 and 25(OH)D3 in human serum. In addition, the researchers found the metabolite 3-epi-25(OH)D in the adult human serum samples, which was previously thought to only exist in the blood of infants. \"This reference material provides a mechanism to ensure measurement accuracy and comparability and represents a first step toward standardization of 25(OH)D measurements,\" the researchers said. The findings were published in the journal Analytical Chemistry.