Ankle-Brachial Index (ABI) Measurement 

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

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]

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.

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
 

Periprocedural Care

Preprocedural Planning

In the clinical setting, the patient should plan on wearing loose-fitting clothes that allow access to the the arms and ankles.

Equipment

Equipment used in the measurement of the ankle-brachial index (ABI) includes the following:

  • An appropriately sized blood pressure cuff - The width of the cuff should be at least 40% of the limb circumference; a manual blood pressure cuff is preferred, in that it is less prone to error than automatic cuffs are 
  • A Doppler device for detecting flow
  • Ultrasound transmission gel

Patient Preparation

Anesthesia

No anesthesia is recommended. Any form of sedative or anesthetic may affect the blood pressure measurement and thus reduce the accuracy of the ABI.

Positioning

The patient should lie on an examination table in a supine position. The supine position is preferred to the seated position because it relieves the hydrostatic pressure on the extremities by keeping them on the same plane as the heart.[4, 14] ABI results can increase by up to 0.3 if the examination is performed with the patient in a seated position.[6] The patient should rest for at least 10 minutes in the supine position.

 

Technique

Approach Considerations

Different values obtained for the ankle-brachial index (ABI) are interpreted as follows:[15, 16]

  • 0.00 to 0.40 - Severe peripheral arterial disease (PAD) sufficient to cause resting pain or gangrene
  • 0.41 to 0.90 - PAD sufficient to cause claudication
  • 0.91 to 1.30 - Normal vessels
  • >1.30 - Noncompressible, severely calcified vessel

Patients with an ABI lower than 0.90 have a higher risk of coronary artery disease, stroke, and death and therefore should be referred to a credentialed vascular laboratory for further testing. Such testing may include computed tomography (CT), magnetic resonance imaging (MRI), or conventional angiography, all of which involve injecting contrast material in order to assess the arterial flow.[7]

For accuracy, systolic blood pressure readings should be obtained from both arms, and the higher of the two values—not the average of the two—should be used in the ABI calculation. Two systolic blood pressure readings should be obtained from each ankle, and again, the higher of the two values should be used. 

Claudication is a specific but not a sensitive finding in patients with PAD. One study stated that up to 90% of patients with a documented ABI lower than 0.90 did not report claudication as a symptom.[3, 7, 16]

An ABI of 0.91-0.99 is considered borderline. The patient may be asymptomatic at rest but may experience symptoms related to the compromised vascular flow when ambulating. An exercise test may help evaluate a patient who has borderline ABI results.

An incorrectly performed test may lead to a false-negative or a false-positive result and thereby delay the diagnosis or prompt unnecessary further testing. A false-negative ABI result may occur in patients with noncompressible arteries—that is, a heavily calcified lower-extremity vessel may artificially elevate ankle pressure measurement.

Measurement of Ankle-Brachial Index

Place the patient in the supine position, with the arms and legs at the same level as the heart, for a minimum of 10 minutes before measurement

Select an appropriately sized blood pressure cuff for both the ankle and the arms (see the images below); the cuff width should be, at a minimum, 40% of the extremity circumference. The ankle cuff should go on the leg between the malleolus and the calf. Enough room should be left below both cuffs to permit placement of the ultrasound gel, so that the Doppler device can adequately detect the brachial, dorsalis pedis, and posterior tibial pulses.[7, 16]

Pick appropriately sized cuff on basis of width of Pick appropriately sized cuff on basis of width of lower extremity.
Make sure that cuff completely encircles lower ext Make sure that cuff completely encircles lower extremity.
Make sure you have appropriately sized upper-extre Make sure you have appropriately sized upper-extremity cuff. Note that patient is in supine position.

Obtain the brachial systolic pressures of both arms (see the images below), and choose the higher of the two values as the brachial systolic pressure (the difference between them should be less than 10 mm Hg). The brachial pulse is best appreciated on the medial side of the antecubital fossa.

Do not obscure artery with cuff; you will need to Do not obscure artery with cuff; you will need to be able to palpate artery and/or locate it with Doppler device.
As with each appropriately identified arterial sit As with each appropriately identified arterial site, mark site with ultrasound transmission gel after palpation.
Locate artery with device. Typically, Doppler prob Locate artery with device. Typically, Doppler probe must be positioned at 45-60 degrees, not at 90 degrees as shown.

Obtain the anterior tibial and posterior tibial systolic pressures of the extremity in question (see the images below), and select the higher of the two values as the ankle pressure measurement. The posterior tibial pulse is best appreciated just dorsal and inferior to the medial malleolus. The dorsalis pedis pulse is best appreciated on the dorsum of the foot between the proximal section of the first and second metatarsals, usually above the navicular bone.

Palpate artery. Dorsalis pedis is palpated by hand Palpate artery. Dorsalis pedis is palpated by hand before Doppler device is used.
Place small amount of ultrasound transmission gel Place small amount of ultrasound transmission gel at landmark where artery was located.
Locate artery with Doppler device. Locate artery with Doppler device.
Posterior tibial artery is located and marked with Posterior tibial artery is located and marked with gel.
Identify artery with Doppler device. Upon applicat Identify artery with Doppler device. Upon application of Doppler probe, arterial pulsations should be audible. If they are not, reposition probe until appropriate sound is obtained.

Finally, divide the ankle systolic pressure by the brachial artery systolic pressure; the result is the ABI.

Alternative techniques for measuring the ABI have been reported. Ichihashi et al assessed the diagnostic ability and reproducibility of a four-cuff automated oscillometric device as an alternative to the Doppler method for obtaining ABI measurements.[8]  With Doppler ABI measurement as the reference, oscillometric ABI measurement was found to have a sensitivity of 89.1%, a specificity of 94.4%, an accuracy of 94.1%, a positive predictive value of 91.8%, and a negative predictive value of 92.4%. Hageman et al also found oscillometric ABI measurement to have good diagnostic accuracy for PAD but noted the substantial incidence of oscillometric errors.[14]