What is the principle of LDH assay?

LDH (Lactate dehydrogenase) assay is a cytotoxicity or cell death assay. It measures the enzyme activity of LDH in a sample to assess the extent of plasma membrane damage in a cell population. 

LDH is a stable enzyme that is present in all cell types. It is rapidly released into the cell culture medium when the plasma membrane is damaged. LDH assay is based on the ability of LDH to generate NADH by oxidizing lactate. The NADH that’s generated reacts with WST (water soluble tetrazolium) to produce a yellow color. The intensity of the yellow color correlates directly with the number of lysed cells.