The superoxide dismutase (SOD) assay is a quantitative measurement of the activity or concentration of the SOD enzyme in blood serum, red blood cells, or other tissues. SOD is a key antioxidant enzyme in the first line of defense against reactive oxygen species (ROS), catalyzing the conversion of the toxic superoxide anion to hydrogen peroxide and oxygen. In humans, there are three main forms: SOD1 (cytoplasmic), SOD2 (mitochondrial), and SOD3 (extracellular). The assay typically measures the total activity of SOD.