We observed 63 samples with high plasma homocysteine (>50 μmol/L) in the group of patients without nitrous oxide consumption and corresponding to 42 vitamin deficiencies (B6, B9, or B12 deficiencies), 8 inherited diseases (CBS deficiencies), 7 methylenetetrahydrofolate reductase (MTHFR) polymorphisms, and 6 kidney diseases. Here, MTHFR is linked to homocystinuria.