ABTS is considered a safe and sensitive substrate compatible for horseradish peroxidase (HRP) based ELISA assays. In the presence of hydrogen peroxide and HRP, ABTS is oxidized to a radical cation with an adsorption maxima at 820 nm, 734 nm, 650 nm and 405 nm. The latter frequency demonstrates a significant molar extinction coefficient and is generally employed for most ABTS assays. Stopping the reaction with acid does not alter the 405 nm spectrum allowing kinetic and endpoint methods to be measured at the same wavelength.