A vitamin E (primarily alpha-tocopherol) assay measures the level of this group of fat-soluble compounds with antioxidant activity in serum or plasma. Vitamin E acts as a major antioxidant, protecting fat cell membranes from lipid peroxidation caused by free radicals, supporting immune function, and contributing to cell signaling and gene expression. Alpha-tocopherol is the most biologically active form of vitamin E in humans.