What is a homocysteine test?

A homocysteine test measures the amount of homocysteine in a sample of your blood. Homocysteine is an amino acid. Amino acids are molecules that your body uses to make proteins.

Normally, your homocysteine levels are low. That's because your body uses vitamin B12, vitamin B6, and folic acid (also called folate or vitamin B9) to quickly break down homocysteine and change it into other substances that your body needs. High levels of homocysteine in your blood may be a sign that this process isn't working right or that you're lacking certain B vitamins.

If homocysteine isn't broken down, it can build up in your arteries (blood vessels that carry oxygen-rich blood from your heart and lungs to your body). This can not only damage your arteries but also increase your risk of forming blood clots. An increased risk of forming blood clots may increase your risk for heart attack, stroke, and other heart diseases and blood vessel disorders.

Other names: total homocysteine, plasma total homocysteine

What is it used for?

A homocysteine test is most often used to measure your vitamin B6, B12, and folic acid levels. Because these three vitamins break down homocysteine, not having enough of any of them can cause homocysteine levels to increase. Not having enough B6, B12, and folic acid can also lead to serious health conditions such as:

• Anemia (a condition where you don't have enough healthy red blood cells).
• Nerve damage throughout your body.
• Cognitive decline (the loss of memory, reasoning, and thinking skills over time).

A homocysteine test may also be used to:

• Help diagnose homocystinuria. Homocystinuria is a rare, genetic disease that prevents your body from making an enzyme that helps to break down homocysteine. An enzyme is a protein that speeds up certain chemical reactions in your body. Symptoms usually show up in the first year of life, but they may not appear until childhood or later. Common symptoms include problems with eyesight, blood clots, and weak bones. In the U.S., most newborns have a routine screening test to check for homocystinuria.
• Better understand your risk for heart attack or stroke if you already have an increased risk. Your health care provider may order a homocysteine test if you have been diagnosed with heart or blood vessel disease or if you have conditions that increase your risk for heart and blood vessel disease, such as high blood pressure, high cholesterol, and diabetes.

Routine homocysteine testing to screen for heart disease risk isn't recommended for everyone. That's because it's uncertain how much homocysteine levels affect heart and blood vessel diseases. Even though high levels of homocysteine can damage arteries, studies have shown that in most cases lowering homocysteine levels doesn't reduce the risk of heart attack or stroke.

Why do I need a homocysteine test?

You may need this test if you are at high risk for low levels of vitamin B12 or folic acid or if you have symptoms that suggest you lack either of these vitamins. The symptoms may be very mild to severe and may include:

• Dizziness.
• Fatigue and/or weakness.
• Headache.
• Heart palpitations (racing or pounding heart).
• Changes in the color of your skin or fingernails.
• Sores on your tongue or in your mouth.
• Tingling or numbness in your hands, feet, arms, and/or legs.

You may be at high risk for low levels of vitamin B12 or folic acid if you:

• Have malnutrition.
• Are an older adult. Older people often can't absorb enough vitamin B12 from food.
• Have alcohol use disorder or a drug addiction.

Your provider may also recommend this test if you have:

• Had a heart attack or stroke.
• One or more conditions that increase your risk for heart attack or stroke, such as high LDL "bad" cholesterol or high blood pressure.
• Homocystinuria. Because this condition makes the breakdown of homocysteine difficult, you are more likely to have high levels of this amino acid.

What happens during a homocysteine test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You may need to fast (not eat or drink) for 8-12 hours before a homocysteine test. Some medicines and supplements may affect your test results. So, tell your provider about all medicines and supplements you take, especially vitamin B. But don't stop taking any medicines unless your provider tells you to.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

A high homocysteine level may be a sign that:

• You are not getting enough vitamin B12 or folic acid in your diet.
• You (or your child) have homocystinuria. You will probably need more testing to rule out or confirm a diagnosis of homocystinuria.
• You may have a higher risk of heart disease, stroke, or other blood vessel disorders.

Higher than normal homocysteine levels may also happen with other conditions, such as osteoporosis, chronic kidney disease, hypothyroidism, schizophrenia, or Alzheimer's disease or other types of dementia.

If your homocysteine levels are high, it doesn't always mean you have a medical condition that needs treatment. Your results may be affected by:

• Your age. Homocysteine levels may get higher as you get older.
• Your sex. Males usually have higher homocysteine levels than females, but levels in females increase after menopause.
• Smoking.

If you have questions about your results, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a homocysteine blood test?

If you have high homocysteine levels, your provider may suggest that you take vitamin supplements or make changes in the foods you eat. Eating a balanced diet can help you get the right amount of vitamins.

Your provider may also recommend that you take certain vitamins and supplements (known as homocysteine-lowering therapy) if you have homocystinuria or signs of cognitive decline. Research shows that this may help slow down cognitive decline and reduce your risk of heart attack or stroke if you have homocystinuria.