The ankle-brachial index (ABI) is an efficient tool for objectively documenting the presence of lower-extremity peripheral arterial disease (PAD). [1, 2, 3, 4] It is a simple, reproducible, and cost-effective assessment that can be used to detect lower-extremity arterial stenosis in the primary care setting, as well as to identify patients at increased risk for lower-extremity arterial injury after penetrating or blunt trauma.
An ABI less than 0.90 has been shown to have a sensitivity of 90% and a specificity of 98% for detecting a lower-extremity stenosis of greater than 50%. [5, 6, 4] In the trauma setting, an ABI less than 0.90 has been shown to have a sensitivity exceeding 87% and a specificity exceeding 97% for identifying lower-extremity arterial injury. 
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).
In the primary care setting, an ABI is useful in the following 2 settings:
In a symptomatic patient, to diagnose PAD
In an asymptomatic patient, to assess the vascular risk for PAD
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,  as follows:
An ABI less than 0.90 suggests a need for further vascular imaging: angiography in a stable patient, and operative exploration in an unstable patient
An ABI greater 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
Patients who are unable to remain supine for the duration of the examination are not candidates for an adequate ABI. ABI measurement is also contraindicated in a patient in whom the use of an occlusive sphygmomanometer cuff may worsen the extremity injury.
Necessary pieces of equipment for measurement of the ankle-brachial index (ABI) include the following:
An appropriately sized blood pressure cuff for the upper and lower extremities with a working sphygmomanometer
A Doppler device for detecting flow
Ultrasound transmission gel
An examination table
No anesthesia is recommended. Any form of sedative or anesthetic may affect the blood pressure measurement and thus reduce the accuracy of the ABI.
The patient should lie on an examination table in a supine position.
0.00-0.40: Severe peripheral arterial disease (PAD) sufficient to cause resting pain or gangrene
0.41-0.90: PAD sufficient to cause claudication
0.91-1.30: Normal vessels
> 1.30: Noncompressible, severely calcified vessel
Patients with an ABI less 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. 
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 of less than 0.90 did not report claudication as a symptom. [11, 4, 10]
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, 20% greater than the diameter of the extremity. 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. [4, 10]
Obtain the brachial systolic pressures of both arms (see the images below), and choose the higher of the 2 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.
Obtain the anterior tibial and posterior tibial systolic pressures of the extremity in question (see the images below), and select the higher of the 2 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.
Finally, divide the ankle pressure by the brachial artery pressure; the result is the ABI.