Measurement of F₂-isoprostanes (F₂-IsoPs) has been independently verified as one of the most reliable approaches to assess oxidative stress in vivo. However, the rapid clearance of F₂-IsoPs makes the timing of sample collection critical for short-lived oxidative insults. Isoketals (IsoKs) are γ-ketoaldehydes formed via the IsoP pathway of lipid peroxidation that rapidly react with lysyl residues of proteins to form stable protein adducts. Oxidative stress can also activate cyclooxygenases to produce prostaglandin H₂, which can form two specific isomers of IsoK—levuglandin (LG) D₂ and E₂. Because adducted proteins are not rapidly cleared, IsoK/LG protein adduct levels can serve as a dosimeter of oxidative and inflammatory damage over prolonged periods of time as well as brief episodes of injury. Quantification of IsoK/LG protein adducts begins with liquid-phase extraction to separate proteins from lipid membranes, allowing measurement of both IsoK/LG protein adducts and F₂-IsoP from the same sample if desired. IsoK/LG-lysyl-lactam adducts are measured by liquid chromatography tandem mass spectrometry after proteolytic digestion of extracted proteins, solid-phase extraction and preparative HPLC.