Limitation of the MTT and XTT assays for measuring cell viability due to superoxide formation induced by nano-scale TiO2

► MTT and XTT are widely used as objective indicators of cell viability in toxicology. ► Both can be reduced to their absorbant forms by superoxide. ► Nanosized TiO2 induces superoxide formation in different mammalian cells. ► MTT/XTT for measuring cell viability by nanosized TiO2 can yield inaccurate results.