MPO is a white blood cell-derived inflammatory enzyme and measures disease activity from the luminal aspect of the arterial wall. Briefly, when the artery wall is damaged, or inflamed, MPO is released by invading macrophages where it accumulates. MPO mediates the vascular inflammation that propagates plaque formation and activates protease cascades that are linked to plaque vulnerability. White blood cell activation in the bloodstream, in response to luminal injury of the artery wall including fissures, erosions or a degrading collagen cap, leads to MPO release in the bloodstream. Myeloperoxidase (MPO)is a vascular-specific inflammatory enzyme released by white blood cells into the bloodstream in response to vulnerable plaque, erosions, or fissures in the artery wall. Elevated MPO levels are associated with risk of cardiac events in subgroups otherwise characterized as low risk, and may assist cardiovascular risk prediction when used independently or alongside standard biomarker testing such as hsCRP.