The ankle brachial index, or ABI, is a simple test that compares the blood pressure in the upper and lower limbs. Health care providers calculate ABI by dividing the blood pressure in an artery of the ankle by the blood pressure in an artery of the arm. The result is the ABI. If this ratio is less than 0.9, it may mean that a person has peripheral artery disease (PAD) in the blood vessels in his or her 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, though.
During an ankle brachial index test, you lie on your back. A technician takes your blood pressure in both of your arms using an inflatable cuff, similar to the one used in the doctor’s office. The technician also measures the blood pressure in the ankles. The doctor uses these values to compute your ABI.
Your healthcare provider might want you to have an ABI test if you are at risk for PAD. The ABI test can:
- Diagnose PAD and 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 one additional risk factor, such as smoking or high blood pressure
Your healthcare provider also might recommend 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. Your provider might order this test every year, to see if your condition 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 leg. Sometimes healthcare providers use ABI to assess your risk of future heart attack or stroke.
For most people, there are no risks associated with having an ABI test. This test is not recommended 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 follow a normal diet on the day of the test. You shouldn’t need to stop taking any medicines before the procedure.
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 during the procedure.
- A technician will place a blood pressure cuff just above your ankle.
- The technician will place an ultrasound probe over the artery. He or she will use this to listen to the blood flow through the vessel.
- The technician will inflate the blood pressure cuff. He or she will increase the pressure until the blood stops flowing through the vessel. This may be a little uncomfortable, but it won’t hurt.
- 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.
- Next, 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 healthcare 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 this value.
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:
- Stopping smoking
- Treating high blood pressure, high cholesterol, and diabetes, if needed
- 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 to your provider about what your ABI value means 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 will you get the results
- Who to call after the test or procedure if you have questions or problems
- How much will you have to pay for the test or procedure
January 11, 2018
Diagnostic evaluation of chronic venous insufficiency. UpToDate., Screening for lower extremity peripheral artery disease. UpToDate., Noninvasive diagnosis of arterial disease. UpToDate
Sudheendra, Deepak, MD , Fraser, Marianne, MSN, RN