What are the commonly used methods for measuring cytotoxicity?

Commonly used methods for measuring cytotoxicity include:

• ATP assay is based on the principle that viable cells typically have higher ATP (adenosine triphosphate) levels than damaged or dead cells. In this cytotoxicity method, the amount of intracellular ATP is quantified as a measure of cell viability. A drop in ATP levels are indicative of cytotoxicity.
• MTT assay involves the use of MTT, a yellow tetrazolium salt. In this technique metabolic activity is determined by measuring the reduction of MTT to a purple formazan derivative by active cells. The amount of formazan generated corresponds with the number of viable cells. This provides researchers with an indirect measure of cytotoxicity. 
• In the colony formation assay, the capacity of single cells to proliferate and generate colonies in the presence of a test substance is measured. A decline in the number of colonies generated suggests potential cytotoxicity.  
• LDH release assay - LDH or lactate dehydrogenase is a cytoplasmic enzyme. In this technique LDH is incorporated into the culture medium and LDH activity is measured to assess cellular damage or death. Elevated LDH levels suggest cytotoxicity.
• Annexin V/propidium iodide staining uses fluorescent dyes to determine the proportion of apoptotic and necrotic cells in a sample. The fluorescent dye Annexin V labels cells undergoing apoptosis while the fluorescent dye propidium iodide labels cells with compromised membrane integrity. Researchers analyze the fluorescence intensity of each dye to determine the proportion of apoptotic and necrotic cells and assess the cytotoxic effects of a test substance.