Testing for MTHFR variants involves either a genetic test that analyzes your DNA or a blood test that measures homocysteine levels, and often both are used together to get the full picture. The genetic test looks for two specific variants, called C677T and A1298C, while the blood test reveals whether those variants are actually affecting your body’s ability to process folate and clear homocysteine.
What the Genetic Test Looks For
The MTHFR gene carries instructions for an enzyme that helps your body convert folate into its active, usable form. When the gene has certain changes, that enzyme works less efficiently. The genetic test checks for two well-studied variants: C677T and A1298C. You inherit two copies of each gene (one from each parent), so your results will show whether you carry zero, one, or two copies of each variant.
The combination matters. People who are homozygous for C677T (meaning they have two copies, sometimes written as “TT”) have roughly 30 to 50% less enzyme activity than people with no variant copies. The A1298C variant has a milder effect, reducing activity by about 10 to 20%. Having one copy of C677T plus one copy of A1298C (compound heterozygous) can also raise homocysteine levels. But having just a single copy of either variant on its own is unlikely to cause health problems.
The Homocysteine Blood Test
A genetic test tells you what variants you carry, but it doesn’t tell you whether those variants are causing a problem right now. That’s where a homocysteine blood test comes in. Homocysteine is an amino acid that builds up when your body can’t process folate efficiently. Elevated levels are the main clinical consequence of reduced MTHFR function.
Your doctor will typically order a fasting total plasma homocysteine test. Fasting gives a more accurate reading because food intake (especially folate and B12) temporarily lowers homocysteine. If your levels come back elevated and your genetic test shows two copies of C677T, or one copy each of C677T and A1298C, those gene changes are probably the cause. If you have two copies of A1298C but no C677T variants, your elevated homocysteine is likely coming from something else entirely.
This is an important distinction: people with the TT genotype for C677T who get adequate folic acid have blood folate levels only about 16% lower than people with no variants. So even with the highest-risk genotype, folate status and homocysteine levels depend heavily on diet and supplementation, not just genetics.
How to Get Tested
There are two main routes: through a healthcare provider or through a direct-to-consumer kit.
If you go through your doctor, they’ll order the MTHFR DNA assay from a clinical lab. The sample is usually a simple blood draw or a cheek swab. Many providers will order a homocysteine level at the same time, since the genetic result alone doesn’t tell you much without it. Clinical-grade tests are processed using techniques like PCR (a method that copies and analyzes specific DNA segments) and are held to strict accuracy standards.
At-home genetic testing kits are also available. These typically use a saliva sample or cheek swab that you collect yourself and mail back. Some consumer genomics companies include MTHFR variants in their health reports. The trade-off is that at-home tests may not be as accurate as clinical lab tests, and the results usually don’t come with a homocysteine measurement or medical interpretation. If an at-home test flags an MTHFR variant, following up with a clinical homocysteine test through your doctor gives you the most useful information.
Who Actually Needs This Test
MTHFR testing is not recommended as a routine screening test for the general population. The variants are extremely common (up to 10 to 15% of some populations are homozygous for C677T), and carrying a variant doesn’t automatically mean you’ll have health issues. Most major medical organizations do not include MTHFR testing in standard workups for conditions like recurrent miscarriage or cardiovascular disease, because the evidence linking the gene variant alone to these outcomes is conflicting.
Testing makes the most clinical sense in a few specific situations: when you have unexplained elevated homocysteine levels and your doctor wants to find the cause, when a close family member has a known MTHFR variant with documented high homocysteine, or when an infant or child shows signs of severe MTHFR deficiency (a rare condition marked by extremely high homocysteine and low methionine). For the rare severe form, testing in infancy is critical because it’s a true metabolic disorder, not just a common gene variant.
Understanding Your Results
Results are reported as your genotype for each variant. Here’s what the main combinations mean in practical terms:
- 677 CC (no variant copies): Normal enzyme function at this position. No impact on folate processing from this variant.
- 677 CT (one copy): Mildly reduced enzyme activity. Unlikely to raise homocysteine on its own, especially with adequate folate intake.
- 677 TT (two copies): Enzyme activity reduced by 30 to 50%. More likely to have elevated homocysteine, particularly if folate intake is low.
- 1298 AC or 1298 CC: Modestly reduced enzyme activity (10 to 20%), but not enough evidence to show this variant alone significantly affects folate processing.
- One copy of C677T plus one copy of A1298C: Compound heterozygous. Can raise homocysteine levels in a pattern similar to 677 TT.
The key takeaway is that genotype alone doesn’t determine your health risk. Two people with the same TT result can have very different homocysteine levels depending on their folate and B12 intake. That’s why the homocysteine blood test paired with the genetic test gives the clearest picture.
What Happens After Testing
If your results show an MTHFR variant with confirmed elevated homocysteine, the most common next step is increasing your intake of folate and B vitamins. Some people are advised to use methylfolate (the already-active form of folate) rather than standard folic acid, since the whole point of the MTHFR enzyme is to convert folic acid into that active form. Your provider may recheck homocysteine levels after a few months to see if they’ve come down.
For people with the TT genotype and elevated homocysteine, there is a mildly increased risk of venous blood clots and recurrent pregnancy loss. Notably, previously suspected links to cardiovascular disease have not held up in more recent evidence. If your homocysteine is normal, even with two variant copies, the clinical significance drops considerably, and no treatment changes are typically needed.