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The ankle brachial pressure index (ABPI) is a ratio which is calculated for the purpose of screening a patient for peripheral artery disease (PAD). It compares the blood pressure taken in the brachial artery of the arm with the blood pressure measured in the ankle or foot. This test is painless and noninvasive, and can be a useful diagnostic tool if a doctor suspects that a patient may have peripheral artery disease. There are some patients in whom this test can return a false negative, however.
To calculate the ankle brachial pressure index, a doctor or nurse takes the patient's blood pressure in his arm, a procedure which many patients may be familiar with, and then repeats the test on the ankle. The ankle blood pressure is divided by the arm blood pressure to return a value. Values of 1.0 to 1.1 are indicators that the patient does not have peripheral artery disease. As the ankle brachial pressure index values drop below 1.0, it shows that the patient may be at risk or has peripheral artery disease, depending on how low the reading is.
This test is usually performed first while the patient is at rest, and then the patient is asked to use a treadmill for a few minutes so that the test can be repeated. In patients who do not have peripheral artery disease, the values should be the same with the second reading. If a patient is at risk or has peripheral artery disease, the ankle brachial pressure index after the treadmill session will be lower, showing that the blood is having difficulty reaching the legs.
There are several conditions which can obscure the test results. These include undiagnosed peripheral artery disease in the arm, which will cause the blood pressure reading in the arm to be lower than it would be in a healthy person. Conditions which lead to hardening or calcification of the arteries can also interfere, as they lead to elevated blood pressure in the legs and thus return a false negative result.
If this diagnostic test reveals that a patient is at risk or has developed partial blockage in the arteries which supply the legs, the doctor can discuss treatment options. In patients who are simply identified as being at risk with their ankle brachial pressure index, some lifestyle changes may resolve the issue and prevent the onset of vascular disease. In patients who are already experiencing arterial blockages, there are treatment options available to address the blockages.
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