Neutral Red Cell Cytotoxicity Test is one of the commonly used methods to detect cell viability or cytotoxicity of drugs. The principle of this test is based on the detection of viable cells via the absorption of the neutral red dye. Neutral red is a eurhodin dye that stains lysosomes in viable cells. Viable cells can absorb neutral red via active transport and incorporate the dye into their lysosomes, but nonviable cells can not absorb this chromophore. Therefore, after washing, the viable cells can release the incorporated dye under acidified extraction conditions.

The amount of dye released can be used to determine the total number of viable cells or the cytotoxicity of the drug. The neutral red absorption test provides a quantitative measure of the number of viable cells and can be measured at an OD of 540 nm.