What is the principle of MTT assays?

MTT assays are a type of colorimetric assay used for determining cytotoxicity or cell viability. MTT (3-(4,5-dimethylthiazol-2-yl)-2–5-diphenyltetrazolium bromide) is cell-permeable and can readily penetrate the intact plasma membranes of viable cells. 

MTT assays are based on the principle that only viable cells that have active metabolism are capable of converting MTT to purple formazan. Purple formazan has an absorbance maximum at 570 nm. A spectrophotometer is used to measure the amount of purple formazan produced, which is proportional to the number of viable cells. Dead cells do not exhibit a visible color change as they do not have any metabolic activity and are unable to convert MTT to purple formazan.