Improved detection of advanced oxidation protein products in plasma.

Oxidative stress has been associated with many diseases and can among others be assessed as increased levels of advanced oxidation protein products (AOPP). Current AOPP methods suffer from poor reproducibility and accuracy due to precipitation of lipids in plasma samples. We therefore aimed to develop a robust method in which plasma lipids are solubilized.Plasma was diluted with citric acid, and AOPP measured as absorbance at 340 nm. The method was optimized and validated, and then used to analyze AOPP levels in plasma from healthy control subjects (HC), and in three patients groups; chronic kidney disease (CKD), primary Sjögren's Syndrome (pSS) and systemic lupus erythematosus (SLE).AOPP was detected with improved precision compared to established methods where lipids precipitate. Within- and between days variations were less than 1.4% and 2.2%, respectively. A control chart was established and the long-term reproducibility followed over six months.This improved method detects plasma AOPP with significantly better reproducibility and accuracy compared to previously reported methods. Solubilization of plasma lipids before spectrophotometric measure of AOPP levels is novel. It prevents both loss of lipoproteins due to precipitation and overestimation as a result of light scattering.