Oxidized LDL is stable in human serum under extended thawed-state conditions ranging from −20 °C to room temperature

INTRODUCTION: Oxidized LDL (oxLDL) is formed by the spontaneous reaction between aldehyde byproducts of lipid peroxidation and lysine residues of apolipoprotein B within LDL. Clinically, oxLDL is used as a marker of coronary artery disease and predictor of metabolic syndrome risk. Despite its popularity as a clinical marker, no systematic studies of oxLDL stability, in which serum or plasma has been pre-analytically exposed to an array of different time and temperature conditions, have been carried out. OBJECTIVE: To systematically evaluate the stability of oxLDL in human serum samples exposed to thawed conditions (> −30 °C) for varying periods of time while monitoring a second protein/small molecule redox system as a positive control for non-enzymatic biomolecular activity. METHODS: OxLDL was measured in serum samples, from 24 different humans, that had been pre-exposed to three different time courses at 23 °C, 4 °C and −20 °C using ELISA kits from Mercodia that employ the 4E6 mouse monoclonal antibody. A liquid chromatography/mass spectrometry-based marker of serum exposure to thawed conditions known as ΔS-Cys-Albumin was employed as a positive control. RESULTS: OxLDL was stable in serum exposed to 23 °C for up to 48 h, 4 °C for 21 days, or −20 °C for 65 days. ΔS-Cys-Albumin changed dramatically during these time courses (p < 0.001). CONCLUSIONS: OxLDL is remarkably stable ex vivo in human serum samples exposed to thawed conditions.