Cell death or cytotoxicity is typically evaluated by quantifying plasma membrane lesions. The LDH cytotoxicity assay kits provide a quick and simple method for quantifying cytotoxicity based on measuring the activity of lactate dehydrogenase (LDH) released by damaged cells.

In contrast to many other cytoplasmic enzymes that exist in many cells, either in small amounts (alkaline and acid phosphatase for example) or unstable, LDH is a stable cytoplasmic enzyme present in all cells and rapidly released into the cell culture supernatant after damage to the plasma membrane. The LDH activity can be determined by a coupled enzyme reaction: LDH oxidizes the lactate to pyruvate which then reacts with the tetrazolium salt INT to formazanate. The increase in the amount of formazan produced in the culture supernatant is directly related to the increase in the number of lysed cells. The formazan dye is water soluble and can be detected spectrophotometrically at 500 nm.