This is a test to measure endothelial dysfunction (improper function of the cells that line the blood vessels). Endothelial dysfunction is the earliest detectable stage of cardiovascular disease. The goal of this test is to identify whether or not you have endothelial dysfunction and if present how severe the dysfunction is. It is best to identify and reverse heart disease at this subclinical level (before there are any outwards signs or symptoms). With lifestyle changes endothelial dysfunction can be reversed.