The name 'vitamin C' always refers to the L-enantiomer of ascorbic acid and its oxidized forms. The opposite D-enantiomer called D-ascorbate has equal antioxidant power, but is not found in nature, and has no physiological significance. When D-ascorbate is synthesized and given to animals that require vitamin C in their diets, it has been found to have far less vitamin activity than the L-enantiomer.
Simple tests use dichlorophenolindophenol, a redox indicator, to measure the levels of vitamin C in the urine and in serum or blood plasma. However these reflect recent dietary intake rather than the level of vitamin C in body stores. Reverse phase high performance liquid chromatography is used for determining the storage levels of vitamin C within lymphocytes and tissue. It has been observed that while serum or blood plasma levels follow the circadian rhythm or short term dietary changes, those within tissues themselves are more stable and give a better view of the availability of ascorbate within the organism. However, very few hospital laboratories are adequately equipped and trained to carry out such detailed analyses, and require samples to be analyzed in specialized laboratories.