Homocysteine is an amino acid produced by the body during the breakdown of proteins. It is normally converted into other substances with the help of B vitamins, including folate (vitamin B9), vitamin B6, and vitamin B12. This blood test measures the amount of homocysteine in the blood. The test is used to help evaluate homocysteine levels, which may be elevated when there is a deficiency of folate, vitamin B6, or vitamin B12. Higher levels have been associated with cardiovascular risk and other conditions. It may also be used to screen for inherited disorders that affect homocysteine metabolism.