Homocysteine, a nonessential amino acid, participates in methionine and folate cycles. It is defined by an elevated homocysteine level (≥15 μmol/L). Hyperhomocysteinemia may be congenital or acquired. Acquired forms are observed in individuals who lack folate, vitamin B12, or other causes. Mutations in the cystathionine β-synthase, called homocystinuria or 5–10-methylenetetrahydrofolate reductase (MTHFR) genes, typically cause severe congenital hyperhomocysteinemia. It is associated with a higher risk of both arterial and venous thrombosis. The total blood homocysteine (tHcy) level is often measured by chromatographic and immunoassay quantitative methods. Almost all analytical procedures for measuring tHcy involve a series of steps, including reduction, derivatization, separation, and detection. The methionine loading test can detect homocystinuria in patients with hyperhomocysteinemia.