High Homocysteine on a Blood Test: What It Means
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 31 October 2026
High homocysteine on a blood test: normal ranges in µmol/L, common causes such as low B12 or folate, what it means for your heart and what happens next.
You don't have to figure this out alone
Dr. Pedro Yelamo Vera is an independent, bilingual GP licensed in Spain. Book a 15-minute video consultation in English or Spanish for €50, with a private electronic prescription issued under Spanish law when clinically appropriate.
High homocysteine means there is more of this amino acid in your blood than your lab expects. Common reasons are low vitamin B12 or folate, weaker kidney function, smoking and older age. A raised level is linked to heart disease and stroke. But lowering it with vitamins has not been shown to prevent heart attacks.
- Homocysteine is measured in µmol/L. Each lab sets its own upper limit, often somewhere between about 11 and 18 µmol/L.
- B12, folate and vitamin B6 normally clear it. A shortage of any of them makes it rise.
- Kidney disease, an underactive thyroid, smoking and age can also raise it.
- A high level is a marker of heart and stroke risk. B vitamin tablets did not cut heart attacks in trials, says a Cochrane review.
- The useful next step is to find the cause: B12, folate, kidney tests and a blood count.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
What is homocysteine?
Homocysteine is an amino acid, one of the building blocks your body uses for proteins. MedlinePlus explains that vitamin B12, vitamin B6 and folate (folic acid) break it down quickly. So the level in the blood is normally low. When it is high, that process is not working well, or you are short of one of these vitamins.
On a Spanish report it appears as homocisteína or homocisteína total. In the UK and the US it is "homocysteine" or "total homocysteine". The unit is the same in all three: µmol/L.
What is a normal homocysteine level?
There is no single range. Labs differ, so read the range printed next to your result. Two NHS hospital labs show how much this varies:
- North Bristol NHS Trust: 2.0 to 11.2 µmol/L for women, and 2.0 to 14.2 µmol/L for men.
- Leeds Teaching Hospitals: under 18 µmol/L. It calls 18 to 30 µmol/L a mild rise.
MedlinePlus, the US public health site, does not give a set range at all. It notes that levels rise with age, are higher in men, and rise in women after the menopause. Smoking raises them too. A star or an "H" beside your value means it is above your lab's range.
Why is my homocysteine high?
MedlinePlus lists these causes and links:
- Low vitamin B12 or folate. Low vitamin B6 can play a part too.
- Chronic kidney disease. The kidneys clear less homocysteine.
- An underactive thyroid (hypothyroidism).
- Age, male sex, the menopause and smoking.
- Homocystinuria. This is a rare inherited condition. MedlinePlus says symptoms usually start in the first year of life.
Many things lower B12 or folate. The NHS names a vegan diet without supplements, pernicious anaemia, stomach surgery and heavy drinking. It also names some medicines: metformin, acid-reducing tablets (PPIs) and nitrous oxide for B12, and methotrexate, sulfasalazine and some epilepsy medicines for folate. Do not stop any medicine without asking the doctor who prescribed it.
Is high homocysteine dangerous for my heart?
It is linked to a higher risk of heart disease, stroke and other blood vessel problems, says MedlinePlus. It is best seen as a marker, not a cause you can simply switch off.
A Cochrane review pooled 15 trials with more than 71,000 people. B vitamins lowered homocysteine but did not reduce heart attacks or deaths. Strokes were a little less common: 4.3% with vitamins against 5.1% without. MedlinePlus states that lowering homocysteine usually does not cut heart attack or stroke risk.
So the usual risk factors stay the main things to check and treat: blood pressure, cholesterol, blood sugar, weight and smoking. Our guide to high LDL cholesterol covers the main risk check.
Got a high homocysteine result and not sure what it means?
You can book a video visit with a GP licensed in Spain, in English or Spanish. He reads your whole report, checks your B12, folate and kidney results, and explains what to do next.
Spain, the UK and the US: who gets this test?
- US. MedlinePlus says experts do not advise routine homocysteine tests for everyone to judge heart risk. It is used for people with signs of low B12 or folate, those at high risk, or after a heart attack or stroke.
- UK. NHS labs list narrow reasons, such as stroke, homocystinuria and inherited B12 disorders.
- Spain. The test may also appear on a wide check-up panel. A mild rise in someone who feels well does not always mean a problem that needs treatment, as MedlinePlus notes.
Can the test be wrong?
Sometimes. The sample is unstable once taken. North Bristol asks for it to be sent on ice, and Leeds asks labs to separate it within 30 minutes. A delay can affect the result. MedlinePlus says you may need to fast for 8 to 12 hours. Tell the lab about any B vitamins you take. If a result looks odd, a repeat test is reasonable.
What about the MTHFR gene?
MTHFR is a gene that helps the body use folate. Some private tests check it. The CDC says one common variant, C677T, is carried by more people in the US than not. People with two copies have slightly lower folate on average. The CDC says they can still use folic acid normally. It is not a reason to avoid it.
What happens next?
- Check the cause. B12, folate, a blood count, kidney tests such as creatinine, and often a thyroid test.
- Low B12 or folate: treat the deficiency. The doctor checks B12 before giving folic acid alone. Our B12 guide covers other B12 tests, such as MMA.
- Normal vitamins and kidneys: focus on heart risk, quit smoking and repeat the test if needed.
- A very high level, or a young person with clots: a specialist referral.
If you could become pregnant, the CDC advises daily folic acid, whatever your MTHFR result. Our page on folic acid (Acfol) in Spain explains more.
When should you get help now?
See a doctor today for new numbness, pins and needles, balance problems, or a painful swollen calf.
Call 112 for a drooping face, arm or leg weakness, slurred speech, sudden confusion, chest pain, or sudden breathlessness.
Talking it through with a doctor by video
Dr. Yelamo is a GP licensed in Spain. He sees adults, children and pregnant patients by video, every day from 07:30 to 20:30 (Spain time). In the visit he will:
- Read your full blood report, not just the homocysteine line.
- Ask about your diet, drinking, smoking, supplements and medicines.
- Look at your B12, folate, kidney and thyroid results if you have them.
- Explain whether you need a repeat test, more tests or a specialist.
He does not promise any test or prescription before the visit. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.
A prescription is never guaranteed: it is issued only where clinically appropriate after a genuine evaluation, as a private electronic prescription under Spanish law. This service is for urgent, non-emergency care. In an emergency, call 112.
See how the Urgent Care video visit works. It lasts 15 minutes and costs €50.
This guide is general information. It does not replace a consultation with your doctor.
Frequently asked questions
Is high homocysteine serious?
Usually it is a sign to look for a cause, such as low B12 or folate, kidney problems or smoking. It is linked to heart and stroke risk, but on its own it is not an emergency.
What is a normal homocysteine level?
Labs differ. Upper limits range from about 11 to 18 µmol/L. Always read the range printed on your own report.
Do B vitamins lower homocysteine?
Yes, B12, folate and B6 lower it. But a Cochrane review found they did not reduce heart attacks or deaths, and only slightly reduced strokes.
Do I need to fast for a homocysteine test?
You may be asked to fast for 8 to 12 hours. Tell the lab about any B vitamin supplements you take.
Should I get tested for MTHFR?
The common MTHFR variant is very frequent. The CDC says people who carry it can use folic acid normally. Ask a doctor before paying for a gene test.
What is homocisteína on a Spanish blood test?
It is the Spanish name for homocysteine. It is shown in µmol/L, the same unit used in the UK and the US.
Sources
- MedlinePlus: Homocysteine test
- Martí-Carvajal et al., Cochrane: Homocysteine-lowering interventions for preventing cardiovascular events
- NHS: Causes of vitamin B12 or folate deficiency anaemia
- CDC: MTHFR gene variants and folic acid
- Leeds Teaching Hospitals NHS Trust: Homocysteine, total
- North Bristol NHS Trust: Homocysteine (total)
Related guides
Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
