MPO (Myeloperoxidase)

Myeloperoxidase is a heme-containing peroxidase stored in azurophilic granules of neutrophils and monocytes that generates hypochlorous acid and other reactive oxidants during inflammation. Released into plasma at sites of vascular inflammation and plaque rupture, MPO is a marker of leukocyte activation, oxidative stress and atherosclerotic plaque instability. Plasma MPO is typically measured by sandwich ELISA or chemiluminescent immunoassay, with healthy reference values commonly below 350-630 pmol/L (assay-dependent). In Brennan et al. (NEJM 2003), among 604 patients with chest pain and a negative initial troponin, baseline plasma MPO independently predicted myocardial infarction and major adverse cardiovascular events at 30 days and 6 months. Confounders include any acute infection, ANCA-associated vasculitis (MPO is the autoantigen of p-ANCA), heparin therapy (which releases endothelial-bound MPO and falsely raises levels), smoking, and pregnancy.