Ankle-Brachial Index (ABI) Measurement

Updated: Sep 10, 2020
  • Author: Chan W Park, MD, FAAEM; Chief Editor: Karlheinz Peter, MD, PhD  more...
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Overview

Background

The ankle-brachial index (ABI) is a noninvasive diagnostic test that compares blood pressures in the upper and lower limbs to determine the presence of lower-extremity peripheral arterial disease (PAD). [1, 2, 3]  It is a simple, reproducible, and cost-effective assessment that can be used to improve detection of lower-extremity arterial stenosis. ABI outperforms the history and physicial examination in its ability to predict angiographically proven PAD. [4]

Although the ABI can also aid in assessment of postoperative wound healing and ulcer management, it is more commonly used to identify PAD in the clinical setting, where the goal is to screen, diagnose, and treat at-risk patients in order to institute early risk factor modification for coronary vascular disease (CVD). In the acute trauma setting, ABI can help identify and risk stratify those at risk for lower-extremity arterial injury. [4, 5, 6]

For patients with clinical suspicion of significant PAD (stenosis ≥50%), an ABI lower than 0.90 has been reported to have a sensitivity ranging from 79% to 95%, with a specificity exceeding 95%. [7, 8, 9]  In diabetic patients with neuropathy or ulcers, however, the accuracy of ABI in early detection of PAD is much more variable, with sensitivity ranging from 29% to 100% and specificity from 58% to 97%. [10, 11] Accordingly, if clinical suspicion is high for PAD despite a nondiagnostic ABI, additional testing should be pursued. [9, 7]

Another area where ABI has proved useful is in the evaluation of potential arterial injury after acute blunt or penetrating injury to the lower extremity in stable patients. In this setting, an ABI lower than 0.90 has been shown to have a sensitivity exceeding 87% and a specificity exceeding 97% for identifying lower-extremity arterial injury. [5]

Further information on the use of the ABI is available in the PAD management guidelines developed by the American College of Cardiology (ACC) and the American Heart Association (AHA). [12]

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Indications

As a general principle, any test for PAD should only be performed either to confirm its presence or to inform treatment decisions. In the clinical setting, an ABI is primarily useful for the following two purposes:

  • Symptomatic patients - To diagnose PAD
  • Asymptomatic patients - To identify patients at risk for PAD 

The simplicity and low cost of the ABI makes it a convenient diagnostic test for early detection of arterial disease or acute arterial injury; however, it may have limitations in certain patient populations. In a systematic review and meta-analysis aimed at determining the diagnostic accuracy of the ABI for detecting PAD in diabetics as compared with imaging methods (angiography or color duplex ultrasonography [US]), Chuter et al found the ABI to have variable results, with sensitivity ranging from 29% to 100% and specificity from 58% to 97%. [11]  Thus, in diabetics with signs of neuropathy or foot ulcers, a negative ABI result should be interpreted in context, and further testing may be warranted.  

In the emergency or trauma setting, an ABI is useful for the evaluation of a patient who is at increased risk for lower-extremity arterial injury, [13] as follows:

  • An ABI lower than 0.90 suggests a need for further vascular imaging: angiography in a stable patient, and operative exploration in an unstable patient
  • An ABI higher than 0.90 decreases the likelihood of an arterial injury; thus, the patient may be observed with serial ABI assessments or may undergo a vascular study on a delayed basis

The current utilization of ABI in trauma is limited, as a consequence of the increased use of routine computed tomography (CT) scanning in these settings; it is often deferred in favor of CT angiography (CTA) of the lower limbs, which can be performed while other parts of the body are being imaged.

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Contraindications

Contraindications for obtaining the ABI include the following:

  • Patients who are unable to remain supine for the duration of the examination 
  • Patients in whom the use of an occlusive sphygmomanometer cuff may worsen the extremity injury
  • Patients with a known or suspected DVT - Compression of the extremity should be avoided out of concern for breaking and embolizing a thrombus
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