The MTHFR C677T mutation is a genetic variant in the MTHFR gene, which provides instructions for making an enzyme called methylenetetrahydrofolate reductase. This enzyme plays a crucial role in processing amino acids, the building blocks of proteins, and in converting an amino acid called homocysteine to another amino acid, methionine. Methionine is important for several biochemical processes in the body, including DNA methylation, a process that regulates gene expression.

The MTHFR C677T test is a genetic test that determines whether an individual carries a specific mutation in the MTHFR gene. The MTHFR C677T mutation involves a change in the DNA sequence at position 677, where cytosine (C) is replaced with thymine (T). This results in substitution of the amino acid alanine for valine, producing a less active form of the MTHFR enzyme.

This test is sometimes ordered as a follow-up to an elevated homocysteine test and may be occasionally ordered along with other cardiac risk tests if a person has a personal or family history of premature cardiovascular disease (CVD) or inappropriate blood clots (thrombosis). In pregnant women, testing for the MTHFR C677T mutation might be considered, especially if there is a family history of neural tube defects.