What does an ankle-brachial index (ABI) reveal and when is it indicated?

26th Feb 2021

An ankle-brachial index (ABI) test provides a lot of useful information and is a quick, noninvasive way to check for obstructive peripheral arterial disease (PAD). Before we get into what the ABI represents and when ordering one might be useful, you should be aware that there are two components of an automated ABI report:

  1. The quantitative portion consisting of the ABI ratio.
  2. The qualitative portion consisting of the audible and analog waveforms.

The quantitative portion consists of a ratio of the systolic blood pressure taken in the ankle over the systolic blood pressure taken in the arm. This calculation creates the index value. 

Patient with a systolic blood pressure of 120 mmHg in the arm and 108 mmHg in the ankle. Ankle-brachial index (ABI) equation showing 108 over 120 equals 0.9. Illustration.

Figure 1. The quantitative portion of an automated ankle-brachial index (ABI) report consists of a ratio which is calculated by dividing the systolic blood pressure in the ankle by the systolic blood pressure in the arm. 

The qualitative component of an ABI test consists of the audible and analog waveforms produced by the Doppler on an automated ABI machine. These waveforms, in conjunction with the ratio, are used to classify the presence and degree of blockage. 

It’s important to note that there’s a key difference between manual ABIs (which only require a Doppler pen and blood pressure cuff and pump) and automated ABIs (which require an automated ABI machine). Both the manual and automated methods provide us with ratios and audible waveforms. But, only an automated ABI machine can provide us with analog waveforms. 

The most useful part of analog waveforms is that they can be printed in a report and shared. The waveforms are then correlated with the ABI ratios to help with diagnosis.

Music note and waves indicating audible waveforms. Graph paper with analog waveforms. Illustration.

Figure 2. The qualitative portion of an automated ankle-brachial index (ABI) report consists of audible waveforms, which can only be heard, and analog waveforms, which can be printed in a report.

 

What does the ABI test mean?

The ABI represents the percentage of blood flow that is traveling from the heart to the ankles. Ideally, the index should be 1, which means that 100% of the blood is reaching the ankles. In fact, the index can be slightly greater than 1 due to hydrostatic pressure that naturally increases from standing during the day. 

On the other hand, an ABI of 0.5 means that only 50% of the blood is reaching the ankles, and 50% is blocked by PAD. 

Patient with downward arrow from the heart to the ankle beside text. ABI ratio of 1.0 equals 100%. ABI ratio of 0.5 equals 50%. Illustration.

Figure 3. When performing an ankle-brachial index (ABI) test, the ABI ratio represents the percentage of blood that is reaching the ankles. 

 

When is an ABI indicated?

An ABI can be used to evaluate a patient for suspected occlusive PAD, but not for a patent aneurysm without obstructive mural thrombus. As well, it would be rare to have enough mural thrombus to show up on an ABI unless an aneurysm was occluded by mural thrombus (then acute limb ischemia would happen). 

Keep in mind that PAD is not ruled out by a normal ABI. Rather, the ABI results help differentiate what type of PAD the patient may have.

For example, patients with PAD due to a true aneurysm or even an active pseudoaneurysm will usually have normal ABI results. However, patients who have PAD that is of an occlusive etiology will have abnormal ABI results. Thus, the ABI is mostly used when an occlusive disease is suspected, such as atherosclerosis.

With a patient who has atherosclerotic PAD, one would expect abnormal ABI ratios and abnormal ABI waveforms.

Lower leg with abnormal ankle-brachial index (ABI) ratios and waveforms for the right posterior tibial artery (PTA) and the right dorsalis pedis artery (DPA). Illustration.

Figure 4. A patient with atherosclerotic peripheral arterial disease would be expected to show abnormal ankle-brachial index (ABI) ratios and abnormal waveforms.

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