Homocysteine can be detected by blood sampling in a vein, usually fasting blood. Homocysteine is an intermediate product of the metabolic conversion of methionine in proteins ingested by the body. Normal serum levels are 5-15 μmol/L, and levels above 15 μmol/L are called hyperhomocysteinemia. Fasting blood tests are more accurate and can minimize interference from diet and other sources. Excessive levels of homocysteine can damage vascular endothelial cells and stimulate the growth of vascular smooth muscle cells, as well as disrupting the normal blood clotting mechanism and increasing the risk of thrombosis, which can lead to heart disease and stroke. It is used clinically to predict cardiovascular and cerebrovascular diseases, depression, miscarriage and birth defects. Homocysteine is mainly measured by fasting venous blood, and if elevated, it is recommended to seek medical advice.