The ankle brachial index (ABI) is a simple test that compares the blood pressure in the upper and lower limbs. Healthcare providers calculate ABI by dividing the numbers for the blood pressure in an artery of the ankle by the numbers for the blood pressure in an artery of the arm. The result is the ABI. If this ratio is less than or equal to 0.9, it may mean that a person has peripheral artery disease (PAD) in the blood vessels in their legs.
In PAD, plaque builds up in the arteries. It often affects the vessels that bring blood to the legs. The reduced blood flow can cause pain and numbness. Low ABI may mean that your legs and feet aren’t getting as much blood as they need. An ABI test won’t show exactly which blood vessels have become narrowed or blocked.
During an ABI test, you lie on your back. A technician takes your blood pressure in both of your arms using an inflatable cuff, similar to the type used in the healthcare provider’s office. The technician also measures the blood pressure in the ankles. The provider then uses these values to compute your ABI.
Your healthcare provider might want you to have an ABI test if you're at risk for PAD. The ABI test can:
Diagnose PAD to help prevent its progression and complications
Identify people who have a high risk for coronary artery disease
Things that can increase your risk for PAD include:
Being older than age 70
High levels of lipids in your blood
Known plaque formation in other arteries, like the coronary arteries in your heart
Abnormal pulses in your lower legs
Being younger than age 50, with diabetes and 1 additional risk factor, such as smoking or high blood pressure
Your provider also might advise an ABI if you have symptoms of PAD, like pain in the legs with activity. But not everyone with PAD has symptoms. This makes the test even more important.
You also might need an ABI to check the severity of your PAD. You may need this test every year to see if your PAD is getting worse.
If you’ve had surgery on the blood vessels of your legs, your provider might want an ABI to see how well blood is flowing into the legs. Sometimes healthcare providers use ABI to assess your risk of future heart attack or stroke.
For most people, there are no risks with having an ABI test. This test isn't advised if you have a blood clot in your leg. You might need a different type of test if you have severe pain in your legs.
There is very little you need to do to prepare for an ABI test. You can eat a normal diet on the day of the test. You shouldn’t need to stop taking any medicines before the test.
You may want to wear loose, comfortable clothes. This will allow the technician to easily place the blood pressure cuff on your arm and ankle. You’ll need to rest for at least 15 to 30 minutes before the procedure. Ask if your healthcare provider has any special instructions.
The test is very similar to a standard blood pressure test. Ask your healthcare provider about what you can expect. In general, during your ABI test:
You will lie flat.
A technician will place a blood pressure cuff just above your ankle.
The technician will place an ultrasound probe over the artery. They will use this to listen to the blood flow through the vessel.
The technician will inflate the blood pressure cuff. They will increase the pressure until the blood stops flowing through the vessel. This may be a little uncomfortable.
The technician will slowly release the pressure in the cuff. The systolic pressure is the pressure at which the blood flow is heard again. That is the part of the blood pressure measurement needed for the ABI.
The technician will repeat this process on your other ankle and on both of your arms.
The technician will calculate the ABI. The top number (numerator) is the higher systolic blood pressure found in the ankles. The lower number (denominator) is the higher systolic blood pressure found in the arms.
Sometimes providers will combine an ABI test with an exercise test. You might have an ABI done before and right after exercise to see how exercise changes the results.
You should be able to go back to your normal activities right after your ABI test.
Be sure to follow up with your healthcare provider about your results. In some cases, you may need follow-up testing to get more information about a blocked vessel. This might include an MRI or an arteriogram.
If you have PAD, you may need treatment. Possible treatments include:
Treating high blood pressure, high cholesterol, and diabetes
Staying physically active
Eating a healthy diet
Taking medicine to increase blood flow to your legs or to prevent blood clots
Having procedures to restore blood flow, like angioplasty
Having surgery to your leg if the blockage is severe
Talk with your provider about what your ABI results mean for you.
Before you agree to the test or the procedure make sure you know:
The name of the test or procedure
The reason you are having the test or procedure
What results to expect and what they mean
The risks and benefits of the test or procedure
What the possible side effects or complications are
When and where you are to have the test or procedure
Who will do the test or procedure and what that person’s qualifications are
What would happen if you did not have the test or procedure
Any alternative tests or procedures to think about
When and how you will get the results
Who to call after the test or procedure if you have questions or problems
How much you will have to pay for the test or procedure