Homocysteine is a non-essential sulfur-containing amino acid closely linked to the essential amino acid methionine. In biological samples, homocysteine is rapidly oxidized to homocystine and cysteine–homocysteine disulfide. Total homocysteine represents the sum of homocysteine and its oxidized forms. The determination of total homocysteine from dried blood spot (DBS) samples plays an important role in the diagnosis and management of folate, vitamin B12, and vitamin B6 deficiencies. Elevated total homocysteine has also been identified as an independent predictor of cardiovascular risk.