When the ankle-brachial index (ABI) emerged in 1950, it was initially proposed for use as a noninvasive diagnostic tool for lower-extremity peripheral artery disease (PAD). Since then, studies have shown that the ABI is an indicator of atherosclerosis at other vascular sites, making it a useful prognostic marker for cardiovascular events and functional impairment, even in the absence of symptoms of PAD.
In an issue of Circulation, the American Heart Association (AHA) released a scientific statement with standardized recommendations for measuring and monitoring the ABI. The recommendations provide protocols and thresholds for use in PAD and cardiovascular risk prediction, according to Michael H. Criqui, MD, MPH, FAHA, who co-chaired the writing committee that developed the scientific statement. “A lack of standards for measuring and calculating the ABI can lead to discrepancies that can significantly impact both prevention and treatment of cardiovascular disease,” he says. “The estimated prevalence of PAD may vary substantially according to the mode of ABI calculation.”
Reducing Variation in ABI Technique
Recent studies have revealed that techniques for performing the ABI vary from clinician to clinician. Several variables have been identified, including the position of patients during measurement, the sizes of the arm and leg cuffs, and the method of pulse detection over the brachial artery and at the ankles. Other variables include whether the arm and ankle pressures were measured bilaterally, which ankle pulses were used, and whether a single measure or replicate measures were obtained.
Several recommendations have been endorsed by the AHA for measuring the ABI (Table 1). “These recommendations can serve as a guide to ensure that clinicians are measuring the ABI appropriately,” says Dr. Criqui. “By using evidence-based data to guide how we use the ABI, there is hope we can minimize variations in how the tests are performed.”
Making Interpretations on ABI Readings
The AHA endorsed a threshold ABI score of 0.90 or less for the detection of PAD. When calculating the ABI, clinicians should divide the higher of the two lower extremity pressures by the higher of the right or left arm systolic pressure. Clinicians should use the ABI of the leg with the lower value, except when there are noncompressible arteries.
If the ABI score is higher than 0.90 but PAD is suspected, a post-exercise ABI measurement and other noninvasive tests, which may include imaging, are recommended (Table 2). A score of 0.91 to 1.00 is considered “borderline” and appropriate for further evaluation. For a post-exercise measurement, a drop in ankle pressure of more than 30 mm Hg or decrease of more than 20% in the index should be the diagnostic criterion for PAD. An ABI score higher than 1.40 despite clinical suspicion of PAD should prompt a toe-brachial index or other noninvasive tests (eg, imaging), according to the AHA.
“The ABI is recommended as the first-line non-invasive test for diagnosing PAD if the disease is suspected based on symptoms and clinical findings,” says Dr. Criqui. “In addition, as a prognostic marker, the ABI can add incremental information beyond standard risk scores.” An ABI value of 0.90 or less or greater than 1.40 is abnormal.
Unmet Needs Remain in Interpreting ABI
Several issues have been identified as gaps in evidence for using and interpreting the ABI, according to Dr. Criqui. “It’s still unclear whether specific thresholds should be considered when using the ABI in different sex and ethnic groups. Also, there are few cost-effectiveness analyses available, but studies into this topic are currently under way. In addition, we should carefully investigate potentially easier and faster alternatives for measuring the ABI.”
Importantly, Dr. Criqui adds that it is currently unknown how often ABI measurements should be repeated. “For most patients, the ABI decreases with age and PAD incidence increases,” he explains. “We have limited evidence on the rates of ABI progression and on the cost-effectiveness of repeat measurement of the ABI in different patient groups. It will be important to address these questions as awareness of PAD and its link to cardiovascular diseases improves and use of the ABI increases.”
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