This test measures the amount of homocysteine in blood. Elevated levels of homocysteine in the blood have been related to atherosclerosis, higher risk of coronary heart disease, stroke, peripheral vascular disease and possibly Alzheimer's disease. It helps to identify vitamin B12 deficiency or folic acid deficiency and a rare inherited disease (homocystinuria) that causes a deficiency of one of several enzymes needed to convert food to energy, It also helps to determine a cause for otherwise unexplained blood clots.