MTT, XTT, and resazurin are reduced by mitochondrial metabolic activity to yield coloured or fluorescent products, and thus are useful for and assaying cell viability and quantitating cell number. MTT and XTT are reduced to coloured formazin salts that can be measured by absorbance. MTT generates an insoluble formazin salt, requiring cell lysis before the absorbance can be measured, while XTT does not require cell lysis for measurement. Resazurin is a non-fluorescent blue dye that is reduced to the pink fluorescent compound resorufin, which can be measured by fluorescence or absorbance.
