Homocysteine
Homocysteine is an amino acid your body clears with B vitamins. Higher levels can reflect low B12 or folate, kidney function, age or genetics.
AI-drafted. Awaiting verificationAlso called Hcy, HCY, tHcyLOINC 13965-9
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What it is
Homocysteine is an amino acid your body makes when it processes methionine, a building block found in protein foods. Vitamins B12, B6 and folate (B9) help turn homocysteine back into methionine or into other useful compounds, so levels in your blood normally stay low. The test measures total homocysteine, including the part attached to proteins. The result reflects B vitamin status, kidney function, age, sex, genetics and some medicines.
Why it matters
Homocysteine is mostly a clue about how well your body handles this B vitamin pathway. Higher levels are linked in large studies to more heart attacks and strokes, but the link is modest. Lowering homocysteine with B vitamins has not reduced heart attacks or deaths in trials, and it lowered stroke risk only slightly. For that reason, routine testing to screen for heart disease risk isn't recommended for everyone. Very high levels (above about 100 µmol/L) are rare. Labcorp notes they are typically caused by an inherited condition such as homocystinuria, so they need a clinician's review.
If it’s low
A low homocysteine is common and usually not a concern. It is seen with good B vitamin status, with B vitamin supplements, and during pregnancy. Levels also tend to be lower in younger people and in women before menopause.
Common causes
- B vitamin supplements, especially folic acid
- Pregnancy
- A diet rich in folate and B12, or food fortified with folic acid
- Younger age, and being female before menopause
What can help
- A low result usually needs no action
If it’s high
A higher result is common and often has an ordinary explanation, such as lower levels of vitamin B12 or folate, reduced kidney function, older age or smoking. Some people have higher levels because of inherited differences in how they process homocysteine. Many other things can cause this. Your clinician can put your result in context.
Common causes
- Low vitamin B12, folate or vitamin B6, from diet or poor absorption
- Reduced kidney function
- Older age
- Male sex (levels rise in women after menopause)
- Smoking
- An underactive thyroid
- Certain medicines, including methotrexate, theophylline, levodopa and niacin
- Inherited variants such as MTHFR C677T, mainly when folate is low
- Autoimmune conditions such as lupus
- A blood sample that sat too long before it was chilled and spun, which can read falsely high
- Rare inherited disorders, which cause very high levels
What can help
- Ask your clinician whether vitamin B12, folate and kidney tests would help find the cause
- Eat folate and B12 sources such as leafy greens, beans, eggs, dairy, fish and fortified grains
- Quit smoking if you smoke
- Ask your clinician whether any of your medicines or supplements could play a role
- Ask your clinician whether and when to recheck it
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on age. Choose an age to see the one that applies.
- 18–60up to 14.5 µmol/L
Sources: Labcorp, ARUP Laboratories, MedlinePlus, National Library of Medicine
- 61–70up to 17.2 µmol/L
Source: Labcorp
- 71–80up to 19.2 µmol/L
Source: Labcorp
- 81+up to 21.3 µmol/L
Source: Labcorp
See Homocysteine by age and sex, with the U.S. survey numbers where they exist.
Wellness targets
Narrower ranges some clinicians aim for. Being outside one doesn’t mean something is wrong.
up to 10 µmol/L (Adults). Evidence: Limited. The DACH-LIGA consensus (2003) recommends a target below 10 µmol/L, because in observational studies heart attack and stroke risk rises steadily from about that level, and a 2024 review supports 10 as a cut point. But trials that lowered homocysteine with B vitamins did not reduce heart attacks or deaths, so it is unclear whether the number itself is a good target.
If this applies to you
Homocysteine means a little more for some people. Open the one that fits you.
Vegan or vegetarian
The Academy notes that a mild, early B12 shortage raises homocysteine, often before any symptoms. It is read together with B12 and MMA rather than on its own.
What can affect your result
- Eating: protein can raise the level, so a fasting sample is preferred
- Sample handling: blood must be chilled and spun within about an hour or the result reads falsely high
- Age: levels rise with age, so many labs give age-specific limits
- Sex: men usually have higher levels than women; levels in women rise after menopause
- Vitamin B12, folate and B6 status, and B vitamin supplements
- Kidney function
- Smoking and certain medicines
- Pregnancy, which usually lowers it
- Folic acid fortification of food in your country, which lowers typical levels
- Lab method: reference limits differ between labs
- Blood that is not chilled and spun within about an hour of the draw can read falsely high, because red cells release homocysteine (Labcorp).
- Eating, especially protein, can raise the level, so a fasting sample is preferred.
- Vitamin B12, folate and B6 supplements lower the result; tell your clinician what you take.
- Some medicines raise it, including methotrexate, theophylline, levodopa and niacin (Labcorp).
Usually falls over several weeks once low B12 or folate is corrected. Kidney-related or inherited elevations change little.
When to talk to a clinician
A mildly high result can be reviewed at a routine visit. Talk with a clinician soon if your result is far above your lab's range, and especially if it is above 100 µmol/L, a level that is rare and typically linked to an inherited condition. See a clinician about numbness or tingling in your hands or feet, sores in your mouth, unusual tiredness or memory changes, which can go with low B12 or folate. Get emergency care right away for sudden chest pain, trouble breathing, a swollen and painful leg, or signs of a stroke, whatever your results are.
Questions you could ask
- Does homocysteine add anything to my heart-health or B vitamin picture?
- Should we check my vitamin B12, folate and kidney function?
- Could any of my medicines or supplements be affecting this?
- Should we recheck it in a few months, and does the repeat need a fasting, chilled sample?
- Is this what you would expect given any B vitamins I take?
Questions people ask
Do I need to fast for a homocysteine test?
Usually yes. Labcorp prefers a fasting sample, and MedlinePlus says you may need to fast for 8 to 12 hours. The blood also needs to be chilled and spun within about an hour, so it is best drawn where the sample can be handled right away. Follow the instructions you were given.
Will lowering my homocysteine lower my heart attack risk?
Probably not by much. In a combined review of 15 trials, B vitamins lowered homocysteine but did not reduce heart attacks or deaths, and they cut strokes only slightly. Ask your clinician what makes sense for you.
Why does my lab's upper limit depend on my age?
Homocysteine tends to rise with age, and older adults often absorb less vitamin B12. Labcorp, for example, lists an upper limit of 14.5 µmol/L for ages 18 to 60 that rises to 21.3 µmol/L above age 80. Other labs use different limits, so use the range on your report.
Does an MTHFR gene variant mean my homocysteine will be high?
Not necessarily. The C677T variant can raise homocysteine, mostly when folate intake is low. Your measured homocysteine tells you more directly than the gene result. Ask your clinician whether gene testing adds anything for you.
How do I convert homocysteine from mg/L to µmol/L?
Multiply mg/L by 7.4. For example, 1.5 mg/L is about 11 µmol/L. Most US labs already report in µmol/L.
Show the science and sources
Units
- 1 µmol/L = 1 µmol/L
- 1 µmol/L = 0.1352 mg/L
Open the Homocysteine unit converter
Is a change real?
Homocysteine naturally varies about 7.4% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 23% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Week-to-week biological variation of methylmalonic acid and homocysteine in healthy women (Lindberg M et al.).
For clinicians and researchers
Total homocysteine (tHcy) includes the protein-bound, disulfide and free reduced forms; the assay releases the bound forms before measuring. Labcorp lists an enzymatic method. Pre-analytic handling matters because red cells release homocysteine after the draw: plasma should be separated immediately, or whole blood kept on ice and centrifuged within an hour (Labcorp), and a fasting sample is preferred to set a baseline. Homocysteine sits at the junction of two pathways: remethylation to methionine (vitamin B12 and folate) and transsulfuration to cystathionine and cysteine (vitamin B6, CBS). It is usually read alongside B12, folate and methylmalonic acid, and Labcorp notes the test alone is not meant to diagnose folate or B12 deficiency. Reference limits vary by lab, age, sex, kidney function and folic acid fortification, and severity categories are not uniform across sources: the 2003 DACH-LIGA consensus abstract calls levels above 12 µmol/L moderate hyperhomocysteinemia, while many labs and reviews use 15 µmol/L (ARUP's adult upper limit is 15 µmol/L). Labcorp notes that severe elevation (above 100 µmol/L) is typically caused by an inborn error, most often homozygous CBS deficiency, at about 1 in 300,000 live births. In prospective observational studies, a 25% lower usual level (about 3 µmol/L) was associated with 11% lower ischemic heart disease risk and 19% lower stroke risk (Homocysteine Studies Collaboration 2002). In the 2017 Cochrane review of 15 trials with 71,422 participants, homocysteine-lowering vitamins did not change myocardial infarction (RR 1.02) or all-cause death (RR 1.01) and slightly reduced stroke (RR 0.90).
LOINC 13965-9 (primary), 2428-1. Specimen: serum-or-plasma.
Sources
- Homocyst(e)ine (test 706994), Labcorp
- Homocysteine, Total (test 0099869), ARUP Laboratories
- Homocysteine Test, MedlinePlus, National Library of Medicine
- DACH-LIGA homocystein (German, Austrian and Swiss Homocysteine Society): consensus paper on the rational clinical use of homocysteine, folic acid and B-vitamins in cardiovascular and thrombotic diseases: guidelines and recommendations, Clinical Chemistry and Laboratory Medicine (2003)
- Homocysteine and risk of ischemic heart disease and stroke: a meta-analysis, JAMA (Homocysteine Studies Collaboration) (2002)
- Homocysteine-lowering interventions for preventing cardiovascular events, Cochrane Database of Systematic Reviews (2017)
- Hyperhomocysteinemia and Disease: Is 10 μmol/L a Suitable New Threshold Limit?, International Journal of Molecular Sciences (2024)
- L-Homocysteine (PubChem compound summary), PubChem, National Library of Medicine
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.