How to calculate and interpret ankle-brachial index (ABI) numbers

An ankle-brachial index (ABI) report consists of two parts—the qualitative and quantitative data. The quantitative portion includes the ABI ratios and the patient’s blood pressure measurements from both arms and both ankles. Let’s go over which pressures are measured, how they are used to calculate the ABI ratios, and how to interpret these numbers.

To calculate the ABI ratios, the patient’s blood pressure must be measured in both arms and both ankles. But, the only numbers recorded are for systolic blood pressure. Six values are typically recorded:

1. Right brachial systolic pressure
2. Left brachial systolic pressure
3. Right posterior tibial artery (PTA) systolic pressure
4. Left posterior tibial artery (PTA) systolic pressure
5. Right dorsalis pedis artery (DPA) systolic pressure
6. Left dorsalis pedis artery (DPA) systolic pressure

Brachial artery pressures are chosen because they are the closest to the heart. As well, two ankle systolic pressures are recorded on each side of the body (PTA and DPA).

An automated ABI machine will automatically save the blood pressure values and calculate the patient’s ratios. It will also print analog waveforms. If you are performing a manual ABI, you will need to write all six systolic pressure values down one at a time.

Both the left and right brachial pressures are measured because sometimes there is a contralateral pressure gradient that may indicate subclavian steal syndrome. This is a unilateral occlusion in the subclavian artery that blocks blood flow to the brachial artery and causes lower blood pressure in the affected arm. Usually, this is not considered significant unless there is a 10–20 mmHg difference.

There are circumstances where only a unilateral brachial pressure can be taken, such as the presence of a dialysis shunt, a mastectomy, a lumpectomy, or lymphadenopathy. Always ask the patient if it’s okay to take their blood pressure in both arms. Although unilateral pressure values are not as ideal as bilateral values, it still provides valuable information.

Ankle-brachial index ratios are always performed bilaterally but calculated one leg at a time. There are three steps to calculating the ABI ratio for each leg:

1. Determine the highest brachial pressure (left or right).
2. Determine the highest ankle pressure for each leg (PTA or DPA).
3. Divide the highest ankle pressure on each side by the highest overall brachial pressure.

Step 1: Determine the highest brachial pressure

Choose the highest brachial pressure (regardless of which side it is from) and use that value for both the left and right ABI calculations.

Step 2: Determine the highest ankle pressure for each leg

Two measurements are required for each ankle—the PTA and DPA systolic pressure values. From the ankle pressures recorded, select whichever pressure is higher between the PTA and DPA on each side. These are the values that will be used for the ABI calculations.

Step 3: Divide the highest ankle pressure on each side by the highest brachial pressure

To calculate the left ABI ratio, divide the highest ankle systolic pressure from the left leg by the highest overall brachial systolic pressure. Repeat this process with the highest ankle pressure recorded from the right side to calculate the ABI ratio for the right leg.

If there is a large pressure difference between the brachial and ankle pressures, we can assume there is an obstruction occurring between the two locations.

Once the ABI ratios have been calculated, you can compare these findings with the obtained audible waveforms (e.g., qualitative portion of the ABI report). The sound of the flow you hear from the Doppler probe can help characterize the degree of proximal obstruction.

An ABI ratio between 0.9–1.4 that is correlated with a multiphasic waveform (e.g., triphasic or mild biphasic) is consistent with a patient who does not have arterial occlusion.

An ABI between 0.7–0.9 should correlate with biphasic waveforms (e.g., mild or weak biphasic). These waveforms have a sharp upstroke to the peak and one pit. The findings represent mild to moderate arterial insufficiency. These patients typically present with claudication.

An ABI between 0.3–0.5 represents severe peripheral arterial disease (PAD) and usually correlates with a monophasic waveform (e.g., high or dampened monophasic). These patients typically present with pain and ulcers.

A critical ABI range is less than 0.3 to absent, which correlates with an absent waveform and represents critical PAD. In this case, there is no flow in the DPA or PTA; patients with no flow to the feet typically present with advanced ulcers and gangrene.

Let’s say that you have a patient whose highest right ankle pressure is 139 mmHg and their highest overall brachial pressure is 126 mmHg. The right ABI ratio is calculated by dividing 139 mmHg by 126 mmHg, which equals 1.10 (a normal ABI).

Next, you listen to the patient’s audible waveforms and find them to be triphasic, which correlates with the finding of a normal right ABI and represents normal flow.

Keep in mind that if the patient’s ABI ratio is in the normal range, but the corresponding waveform is found to be monophasic, a further investigation would be needed.

Continuing with our example, let’s say that you had access to the patient’s report from an ABI machine. Automated ABI reports calculate the ratios for you.

In this case, let’s say that the patient’s ABI report shows that the highest ankle pressure on the right is the DPA at 134 mmHg. The ABI machine automatically chooses the patient’s highest ankle pressure and highest overall brachial pressure, which is 126 mmHg, to calculate the ABI ratio. So, the right ABI is calculated by dividing 134 mmHg by 126 mmHg, which equals 1.06 (a normal ABI).

This finding should be correlated with the analog waveforms shown on the patient’s ABI report. If the analog waveforms are triphasic (which is normal), then the patient has normal flow.