A homocysteine test measures the amount of the amino acid homocysteine in the blood. You may have high levels of homocysteine when cholesterol, white blood cells, calcium, and other substances (plaque) build up in your blood vessels. This buildup may increase your chances of having a heart attack, stroke, and blood clots in the lungs (pulmonary embolism) or deep veins of the legs (deep venous thrombosis).

Homocysteine testing may be most useful in checking the overall risk of heart disease for people who have a strong personal or family history of heart disease but who do not have other risk factors that can be controlled, such as smoking or high blood pressure. Homocysteine testing also may be useful for people who have early heart disease but who do not have known risk factors and for people who have had unexplained deep vein thrombosis or stroke.

Why It Is Done

A homocysteine blood test is done to:

• Help determine your risk for having heart disease, stroke, and diseases of the blood vessels (peripheral arterial disease).
• Help identify vitamin B12 deficiency or folic acid deficiency. However, other tests for these deficiencies are available.
• Help identify a rare inherited disease (homocystinuria) that causes a deficiency of one of several enzymes needed to convert food to energy.
• Help determine a cause for otherwise unexplained blood clots.

How To Prepare

Do not eat or drink anything (other than water) for 10 to 12 hours before the test.

Many medicines may affect the results of this test. Be sure to tell your health professional about all the nonprescription and prescription medicines you take.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?) .

How It Is Done

The health professional drawing your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure on the site and then put on a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.