MTHFR Mutation Test: What Your C677T and A1298C Results Mean

An MTHFR mutation shows up on many genetic reports, and the label sounds far more serious than it usually is. MTHFR is a gene that helps your body use folate, a B vitamin, and its two common variants, called C677T and A1298C, are carried by a large share of the population. Most people with one or even two copies never notice anything and never develop a problem linked to them. What usually matters more is your homocysteine level, whether you get enough folate, vitamin B12 and vitamin B6, and why the test was ordered in the first place.

In this article you’ll learn what the MTHFR gene does, how to read each possible C677T and A1298C result, what the evidence does and does not support about blood clots, heart disease and pregnancy, where folic acid and methylfolate fit in, and which blood tests usually tell you more than your genotype.

What the MTHFR gene does

The MTHFR gene carries the instructions for an enzyme called methylenetetrahydrofolate reductase. That enzyme converts one form of folate into the active form your body needs to turn homocysteine, an amino acid, into methionine, which is then used to build proteins, as MedlinePlus Genetics .

Picture one station on a recycling line. When the MTHFR enzyme works more slowly, less active folate is produced, and homocysteine can build up in the blood, particularly when folate or vitamin B12 intake is low. With good folate intake, the slower station is usually well compensated.

Mutation, variant or polymorphism?

Reports use these words interchangeably. Strictly speaking, C677T and A1298C are polymorphisms: common variations found in many healthy people. They are very different from the rare, severe MTHFR mutations, at least 40 of which have been described, that cause homocystinuria, an inherited metabolic disorder usually detected in infancy. In this article, an MTHFR mutation means one of the two common variants, because those are what standard and consumer tests look for.

C677T and A1298C: the two variants on your report

A standard MTHFR test checks two specific spots in the gene. You have two copies of the gene, one from each parent, so each variant can be absent, present once or present twice.

C677T (rs1801133)

At position 677, the DNA letter C is replaced by a T. This change makes the enzyme less stable, especially at higher temperatures, which is why researchers call it thermolabile. Of the two, C677T is the MTHFR mutation with the clearest effect on folate and homocysteine. The CDC reports that people with two copies (677TT) have about 16% less folate in their blood than people without the variant who take in the same amount of folic acid.

A1298C (rs1801131)

At position 1298, an A is replaced by a C. This variant has a milder effect on the enzyme, and even two copies of A1298C on their own are probably not enough to raise homocysteine, according to MedlinePlus. It matters mainly when it appears together with one copy of C677T.

How to read your MTHFR mutation test results

Your report may list genotypes such as 677CT or 1298AC, use the words heterozygous and homozygous, or simply say one copy or two copies. The table below translates each combination into plain language, based on guidance from MedlinePlus and Cleveland Clinic.

Heterozygous vs homozygous

Heterozygous means the variant sits on one copy of the gene; homozygous means both copies carry it. Heterozygous results are extremely common and are not considered a health risk on their own. Homozygous C677T is the result most often linked to higher homocysteine, yet Cleveland Clinic notes that only a small percentage of people with two copies actually develop elevated levels.

Compound heterozygous

Compound heterozygous means one copy of C677T plus one copy of A1298C. MedlinePlus groups this combination with 677TT as one that can raise homocysteine, although the effect is generally milder. Here again, a measured homocysteine value tells you more than the genotype.

How common MTHFR variants are

If your MTHFR mutation result came back positive, you are in large company. Cleveland Clinic estimates that up to half of people have a change in at least one copy of the MTHFR gene, and the CDC states that more people in the United States carry one or two copies of C677T than carry none. Two copies of C677T are found in about 25% of Hispanic people and 10% to 15% of North American white people, according to MedlinePlus Genetics.

Frequencies differ by ancestry, so findings from one population do not always apply to another. A finding shared by roughly half of all people cannot, on its own, explain an individual’s symptoms.

What an MTHFR mutation means for your health

Homocysteine is the result that matters

Homocysteine normally stays low because vitamin B12, vitamin B6 and folate help break it down. A high level can point to a vitamin shortfall, chronic kidney disease, an underactive thyroid or a rare inherited disorder, according to MedlinePlus. To see how labs report and flag different values, read our homocysteine levels guide.

MedlinePlus makes a key point: treatment for high homocysteine is the same whether or not it is caused by a common MTHFR variant. That is why many doctors measure homocysteine directly instead of testing the gene. A normal homocysteine result alongside a positive MTHFR result usually means the variant is not having a measurable effect.

Blood clots and heart disease

MTHFR used to be included in thrombophilia panels, sets of tests that look for inherited clotting tendencies. In 2013, the American College of Medical Genetics and Genomics (ACMG) concluded that meta-analyses, studies that pool the results of many earlier studies, had not confirmed a link between an MTHFR mutation and venous blood clots, nor between high homocysteine and coronary heart disease. Its guidance was clear: MTHFR testing should not be part of a routine clotting workup. The ACMG reviewed the evidence again in 2020 and kept the same position.

If you have had a clot, your care team will focus on other causes and markers; to understand that workup, explore our guía sobre la trombosis venosa profunda.

Pregnancy, miscarriage and neural tube defects

MTHFR comes up constantly in pregnancy forums. Folate is essential early in pregnancy to help prevent neural tube defects such as spina bifida, and the CDC is clear that people with an MTHFR variant can process all types of folate, including folic acid. Folic acid is the only form of folate shown to help prevent these defects, and the CDC recommends 400 micrograms (mcg) a day for anyone who could become pregnant.

Some studies link C677T with recurrent pregnancy loss or preterm birth, but the results vary widely between populations and study designs. The ACMG advises against ordering MTHFR testing as part of an evaluation for recurrent pregnancy loss.

The long list of conditions you may read about online

Online lists tie MTHFR to dozens of problems. Research on most of these links remains mixed, as MedlinePlus Genetics notes, and associations seen in one population often fail to appear in another. Cleveland Clinic states that MTHFR variants most often do not affect health or cause symptoms. A symptom you are living with deserves a proper evaluation, not an explanation borrowed from a common gene variant.

Folic acid, methylfolate and other B vitamins

One of the most persistent claims online is that people with an MTHFR mutation cannot use folic acid and must buy methylfolate (5-MTHF), the already active form. The evidence does not support avoiding folic acid. The CDC states that folic acid intake matters more than your genotype in determining how much folate is in your blood, and Cleveland Clinic says standard prenatal folic acid is enough, with no need for special forms.

Methylfolate is not harmful, and some clinicians prefer it, particularly to bring down a high homocysteine level; MedlinePlus lists it as an option for people who carry a variant. It is a choice, not a requirement. If your doctor measured your folate, you can interpret the number by reading our folate blood test guide.

Folate never works alone. Vitamin B12 and vitamin B6 sit in the same pathway, and a B12 shortfall raises homocysteine and can cause anemia with unusually large red blood cells. High doses of folic acid, above about 1,000 mcg a day, can correct that anemia while hiding the B12 deficiency underneath, which is why B12 is often checked at the same time (Skavinska et al., 2026). To understand those results, review our vitamin B12 blood test guide and consult our vitamin B6 blood test guide. A high red cell size on a blood count can be an early clue; to see why, open our MCV result guide.

Should you get an MTHFR test?

For most people, no. The ACMG, MedlinePlus and Cleveland Clinic all describe routine testing for an MTHFR mutation as unhelpful because the result rarely changes care. MedlinePlus lists a few situations where testing may still be considered:

• A homocysteine test shows high levels and a close relative carries an MTHFR mutation or had early heart or blood vessel disease.
• You are being prescribed methotrexate, a medicine used for some cancers, psoriasis and rheumatoid arthritis.
• A newborn screening test is positive for homocystinuria.

If methotrexate is part of a skin treatment plan, you can find context by visiting our psoriasis symptoms and treatment guide. The test itself is a simple blood draw that needs no special preparation.

Blood tests that tell you more than your genotype

To know whether your folate pathway is working, these tests are usually more useful than a genotype.

Kidney markers are part of a standard metabolic panel; to decode yours, open our comprehensive metabolic panel guide.

¿Cuándo consultar a un médico?

Talk to a doctor if any of the following applies to you:

• Your homocysteine result is above the laboratory’s reference range.
• You are pregnant or planning a pregnancy and are unsure which folate supplement to take.
• You have had a blood clot, or a close relative had a heart attack or stroke at a young age.
• You notice tingling, numbness, unusual tiredness or signs of anemia, which can point to low B12.
• You plan to start or stop high-dose vitamin supplements.

Sudden weakness on one side of the body, facial drooping or trouble speaking needs emergency care; learn the signs by studying our guía de señales de alarma del ictus.

Últimos avances científicos

Studies from the past three years help put a positive result in perspective.

Folic acid still has the strongest pregnancy evidence

A 2026 review in Nutrition Reviews compared folic acid with two active forms, methylfolate and folinic acid. The active forms avoid a build-up of unprocessed folic acid and are less likely to hide a B12 deficiency, yet folic acid remains the only form proven in large randomized trials, studies that assign people to treatments by chance, to prevent neural tube defects. What this means for you: an MTHFR mutation is not a reason to drop folic acid before or during pregnancy; discuss an alternative only if your doctor sees a specific reason.

C677T, not A1298C, shapes the homocysteine response

In a 2023 study of 272 healthy Greek adults with raised homocysteine, three months of either folinic acid or methylfolate lowered levels by a similar amount. People with two copies of C677T saw the largest drop, while A1298C made no difference to the response. What this means for you: when homocysteine is high, B vitamins usually bring it down, and the form matters less than taking them as advised.

Riboflavin and blood pressure: not proven yet

Riboflavin, or vitamin B2, helps the MTHFR enzyme work, and earlier research suggested it might lower blood pressure in people with two copies of C677T. A 2025 Cochrane review, a rigorous summary of all available trials, found only four small trials with 374 participants and judged the evidence very uncertain. What this means for you: riboflavin is not a substitute for proven blood pressure treatment, whatever your genotype.

A variant that may also protect

A 2025 genetic study pooling more than 43,000 people with multiple sclerosis found that C677T carriers had a slightly lower risk of developing the disease, with no effect on how severe it became. The finding is early. What this means for you: common variants are not simply good or bad, which is one more reason not to read a positive result as a disease risk; for background on the condition itself, browse our multiple sclerosis overview.

Pregnancy links depend on the population

A 2025 analysis of 44 studies found an association between C677T and preterm birth in Asian and Indian populations, but not in white European populations, and no meaningful link for A1298C. Studies of this kind show associations, not cause and effect. What this means for you: a positive result does not predict an early delivery, and standard prenatal care and folic acid remain the priorities.

Broad clotting panels add little

A 2026 study of 131 adults referred to a hematology clinic found that only about 1 in 17 inherited clotting panels had been ordered for a guideline-supported reason. The MTHFR A1298C result showed no consistent link with clots, and the useful information came mainly from factor V Leiden and properly confirmed tests for acquired clotting disorders. What this means for you: targeted testing chosen for your history is more informative than a large panel that includes MTHFR.

Glosario

Preguntas frecuentes

Is an MTHFR mutation serious?

Usually not. The common C677T and A1298C variants are found in a large part of the population, and Cleveland Clinic notes that they most often do not affect health or cause symptoms. The exception is the rare, severe MTHFR deficiency that causes homocystinuria, which is a different condition usually found through newborn screening. If you have a positive result, a homocysteine test is the most useful way to see whether the variant has any effect for you.

What does it mean to be heterozygous for C677T?

It means you inherited one copy of the C677T variant and one typical copy. This is one of the most common results, and on its own it is not linked to a meaningful rise in homocysteine for most people. Standard dietary advice and, if you could become pregnant, the usual 400 mcg of folic acid a day apply as for anyone else.

Can I take folic acid if I have an MTHFR mutation?

Yes. According to the CDC, people with an MTHFR variant can process all types of folate, including folic acid, and folic acid is the only form shown to help prevent neural tube defects. Methylfolate is an alternative some doctors suggest, mainly to lower a high homocysteine level, but it is not required. Ask your doctor before taking high doses of any folate supplement.

Should I get tested for MTHFR before pregnancy?

Routine testing is not recommended, and the ACMG advises against it as part of an evaluation for recurrent pregnancy loss. Whatever the result would be, the advice is the same: take folic acid every day before and during early pregnancy. If you have a history of pregnancy complications or blood clots, your doctor will choose the tests that fit your situation.

What are the symptoms of an MTHFR mutation?

The common variants do not cause specific symptoms. Symptoms people attribute to MTHFR, such as fatigue or brain fog, have many other possible causes, some of them easy to check, such as low vitamin B12, iron deficiency or thyroid problems. If you have ongoing symptoms, ask your doctor to look for the cause rather than relying on a genotype result.

Do at-home DNA kits count as an MTHFR test?

Some consumer DNA services report the C677T and A1298C variants, and many people first learn their status this way. The genotype may be accurate, but it carries the same limits as a clinical test: it does not tell you your homocysteine or vitamin levels. If a kit result worries you, a blood test for homocysteine, folate and B12 is a more useful next step than repeating the genetic test.

Fuentes

• Centers for Disease Control and Prevention (CDC) — MTHFR Gene Variant and Folic Acid Facts — reviewed 2026 — cdc.gov
• MedlinePlus, National Library of Medicine — MTHFR Gene Test — MedlinePlus Lab Tests, updated 2026 — medlineplus.gov
• MedlinePlus Genetics, National Library of Medicine — MTHFR gene — updated 2019 — medlineplus.gov
• MedlinePlus, National Library of Medicine — Homocysteine Test — MedlinePlus Lab Tests, updated 2025 — medlineplus.gov
• Cleveland Clinic — What To Know About MTHFR Gene Variants — Health Library, reviewed 2026 — my.clevelandclinic.org
• Hickey SE, Curry CJ, Toriello HV — ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing — Genetics in Medicine, 2013 — doi.org/10.1038/gim.2012.165
• Bashford MT, Hickey SE, Curry CJ, Toriello HV — Addendum: ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing — Genetics in Medicine, 2020 — doi.org/10.1038/s41436-020-0843-0
• Skavinska O, Rossokha Z, et al. — Comparative Analysis of Treatment With Folate Forms in Clinical Practice — Nutrition Reviews, 2026 — doi.org/10.1093/nutrit/nuaf216
• Mazokopakis EE, Papadomanolaki MG, et al. — The effects of folinic acid and l-methylfolate supplementation on serum total homocysteine levels in healthy adults — Clinical Nutrition ESPEN, 2023 — doi.org/10.1016/j.clnesp.2023.09.002
• Bradbury KE, Coffey S, et al. — Riboflavin supplements for blood pressure lowering in adults — Cochrane Database of Systematic Reviews, 2025 — doi.org/10.1002/14651858.CD015464.pub2
• Daghlas I, Pluvinage JV, et al. — Genetically reduced MTHFR activity confers protection against multiple sclerosis — Journal of Neuroimmunology, 2025 — doi.org/10.1016/j.jneuroim.2025.578784
• Vafapour M, Talebi H, et al. — Global and population-specific association of MTHFR polymorphisms with preterm birth risk: a consolidated analysis of 44 studies — BMC Pregnancy and Childbirth, 2025 — doi.org/10.1186/s12884-025-07378-6
• Ozturk Ari A, Pinar IE, Ozkocaman V, Ozkalemkas F — Beyond the Thrombophilia Panel: Real-World Overuse, Limited Interpretability, and Poor Guideline Concordance in a Hematology Referral Cohort — Diagnostics, 2026 — doi.org/10.3390/diagnostics16162643

Lecturas recomendadas

• Explore the many causes of low red blood cell counts by reading our guía sobre síntomas y causas de la anemia.
• See how doctors confirm a clot that has traveled to the lungs by reviewing our guía sobre la embolia pulmonar.
• Learn about an inherited heart risk marker measured by a simple blood test by opening our lipoprotein(a) test guide.
• Decode every line of your routine blood count by exploring our guía del hemograma completo.
• Understand how kidney filtration is estimated from creatinine by visiting our guía del filtrado glomerular estimado (FGe) en sangre.

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An MTHFR result only makes sense next to the numbers that show how your folate pathway is actually working. BloodSense helps you understand your homocysteine, folate, vitamin B12 and complete blood count results in plain language, along with the questions worth asking at your next appointment. It does not make a diagnosis and does not replace your doctor.

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