Malondialdehyde (MDA) is an organic compound (C₃H₄O₂) generated by lipid peroxidation of polyunsaturated fatty acids after degradation by reactive oxygen species (ROS). Lipid peroxidation is a well-established mechanism of cellular injury in plants and animals, used as an indicator of oxidative stress in cells and tissues. Lipid peroxidation products derived from polyunsaturated fatty acids decompose to form a diverse mixture of compounds including MDA. MDA is the most commonly used biomarker of oxidative stress in many health disorders such as cancer, cardiovascular diseases, asthma, and chronic obstructive pulmonary disease.

In addition to MDA, other reactive aldehydes such as 2-alkenals and 2,4-alkedienals, are also formed by these oxidative mechanisms, which can react with Thiobarbituric Acid (TBA) to generate a measurable signal in a quantitative assay. Total MDA combined with other reactive substances are termed as Thiobarbituric Acid Reactive Substances (TBARS). Modifications of the TBARS assay have been widely used to evaluate the MDA levels in different types of samples, including mammalian tissues, serum, plasma and urine along with food samples. The reactivity of acidified TBA towards other reactive aldehydes has led to some sort of ambiguity surrounding the use of TBARS for different sample types under various oxidative stress conditions; nevertheless, the assay is extensively used to determine the level of lipid peroxidation. In general, lipids with increased degree of unsaturation will show higher TBARS values.