Current Insights On ABI And Diagnosing PAD
This author offers pearls on how measuring the ankle brachial index (ABI) can facilitate the accurate diagnosis of peripheral arterial disease (PAD) and help salvage limbs.
How does your podiatry practice test for peripheral arterial disease (PAD)? Blume and colleagues in Podiatry Today reviewed a long list of noninvasive testing options.1 These include ankle brachial index (ABI), toe brachial index, segmental pressure measurements, skin perfusion pressure and laser Doppler pressure, color duplex imaging and ultrasonography, plethysmographic waveform analysis, transcutaneous oxygen content and cutaneous oximetry.
This extensive list might seem daunting but ABI screening stands out as one of the least costly options available. A doctor, nurse, skilled medical assistant or technician can perform ABI testing quickly and easily in an office setting. Therefore, podiatrists should take some time to reconsider their role in diagnosing PAD along with recently updated guidelines for ABI screening.
PAD: A Costly And Underdiagnosed Problem
Peripheral arterial disease is a challenging and serious manifestation of systemic atherosclerotic changes with narrowing of the distal extremity arteries. Timely diagnosis of PAD is of paramount importance to the podiatric physician for improving quality of life and preventing limb loss, morbidity and mortality. A strong marker for cardiovascular disease, PAD is associated with coronary artery disease and cerebrovascular disease.2 Over 10 million people in the United States have PAD with over a 10 percent prevalence in people over 60 years old.3 The cost of treating PAD in the United States is quite high and the estimated cost during 2010 was between $164 and $300 billion.4
The long-term survival of diabetic foot ulcer patients with PAD is especially poor. A recent study in Diabetes Care followed 247 patients with diabetes without previous amputation for a decade.5 The authors found age and PAD were significant predictors for first major amputation with surprisingly high levels of cumulative mortality at one year (15.4 percent), five years (45.8 percent) and 10 years (70.4 percent). The approximate five-year mortality rate of PAD (64 percent) is greater than prostate cancer (9 percent) or breast cancer (14 to 18 percent).6
Despite high costs to society and high levels of mortality, it is amazing how little attention PAD receives in the press and media. Unlike breast cancer, peripheral arterial disease is not often associated with pink ribbons or local 5K races. According to Desmond Bell, DPM, Co-Founder and Executive Director of the Save a Leg, Save a Life Foundation in Jacksonville, Florida, “One of the real issues patients are facing is that PAD is under-diagnosed and under-recognized among practitioners.”7 Although the Southern Arizona Limb Salvage Alliance toe and flow model is gaining acceptance among some practitioners, there is still much we can do for improving PAD diagnosis and treatment.
What Is The Most Effective Way To Measure ABI?
The ABI is the ratio of the ankle pressure (numerator) over the higher brachial pressure from two arms (denominator). However, there are various methods of calculating the numerator. The podiatric physician should be aware of which methods their assistants use to calculate ABI. The most common method is high ankle pressure, which uses the higher of the two systolic ankle pressures as the numerator. Other methods include using the lower ankle pressure, averaging the two results, or reporting only the posterior tibial artery systolic pressure.
An ABI test is a reproducible and fairly accurate measurement of the ankle and brachial systolic pressures. Although vessel calcification can falsely elevate values, ABI testing displays excellent sensitivity and specificity for arterial disease. Khan and coworkers reported that the ABI, in comparison to angiography, has a sensitivity of more than 90 percent and a specificity of more than 95 percent for diagnosing 50 percent stenosis in the lower extremity arteries.8
However, some more recent studies have called this into question. Niazi and colleagues recently reported that high ankle pressure ABI had a sensitivity of 69 percent and specificity of 83 percent.9 Khan and coworkers critically reviewed ABI methods and found low ankle pressure ABI generally has better sensitivity in diagnosing PAD.8 However, high ankle pressure ABI is the current method endorsed by the American College of Cardiology Foundation (ACCF) and the American Heart Association (AHA).10
The ankle brachial index has become the gold standard office-based test to screen for peripheral vascular disease but this test is still underutilized.11 Why is ABI testing often overlooked in primary care settings or podiatry offices?
Perhaps some practitioners believe they should only perform the ABI when patient has a non-healing wound or intermittent claudication. This belief may be further reinforced by the large percentage of tests that come back with normal results. However, it may come as a surprise to some to learn that the majority of patients with PAD are asymptomatic.12 This is an important point to emphasize because screening asymptomatic patients for PAD represents a significant paradigm shift in the way practitioners typically think about diagnosing PAD.
A recent study by Daddato and colleagues calls attention to the excellent opportunity for podiatrists to screen asymptomatic patients with ABI testing.13 The study focused on 749 adults who received PAD screening after presenting to an Italian podiatry clinic. All patients were asymptomatic for PAD and free from cardiovascular diseases. The patients also completed a simple questionnaire for cardiovascular risk factors. Among the entire study population, 8.3 percent of males and 1.2 percent of females had diagnosed PAD. Among those diagnosed with PAD, hypercholesterolemia and hypertension were the highest risk factors reported in 76 percent and 52 percent of patients respectively. The findings of this study were in line with previous studies conducted in other countries but are especially relevant because they were limited to asymptomatic podiatry patients.
A Closer Look At The 2011 ACCF/AHA Recommendations For ABI Screening
When should a podiatrist offer ABI testing to his or her patients? There is no right answer for every podiatry practice. However, the American College of Cardiology Foundation and American Heart Association recently updated their 2005 guidelines for ABI screening and suggest a uniform way of reporting outcomes.14 These 2011 ACCF/AHA guidelines should work well for most podiatry practices.
1. The resting ABI should establish the lower extremity PAD diagnosis in patients with suspected lower extremity PAD, defined as individuals with one or more of the following:
a. Exertional leg symptoms
b. Non-healing wounds
c. Age 65 years or older
d. Age 50 years and older with a history of smoking or diabetes
2. One should measure the ABI in both legs in all new patients with PAD of any severity to confirm the diagnosis of lower extremity PAD and establish a baseline.
3. The ABI results should be uniformly reported in the following categories:
a. Non-compressible >1.40
b. Normal 1.00 to 1.40
c. Borderline 0.91 to 0.99
The 2011 ACCF/AHA guidelines differ significantly from the 2005 guidelines. First, the threshold of 70 years or older decreased to 65 years. Second, the way we report ABI results has been updated with uniform categories. The categories are not complicated and are easy to remember. Additionally, the 2011 guidelines are supported by higher-level evidence than the 2005 guidelines, including data derived from randomized trials.10,14
The ACCF/AHA guidelines are straightforward and evidence-based. They will help eliminate confusion on which patients to offer ABI testing. This consideration is very important in a group practice setting where multiple podiatrists and perhaps physicians of differing specialties need to agree on the standard of care.
One obvious criticism of the ACCF/AHA guidelines is that patients younger than 50 do not get routine screenings unless they present with symptoms or the podiatrist has a high suspicion for PAD.14 Podiatrists should consider ABI screening tests in younger patients based on clinical physical exam findings (missing pulses, missing hair, dependent rubor, thin shiny skin, gangrenous changes, etc.) and clinical symptoms like leg pain that may support a diagnosis of PAD.
At Midwest Podiatry Centers, my fellow podiatrists and I utilize a questionnaire briefly explaining PAD and prompting patients to see if they have had the following symptoms:
• Slow healing wound or ulcers
• Missing pulses or poor circulation
• Injury to legs or extremity
• Exertional cramping or fatigue in extremity relieved by rest
• Resting pain in extremity that may disturb sleep
• Gangrenous black skin tissue
• Toes or feet that have become pale, discolored or bluish
Another criticism of the 2011 ACCF/AHA guidelines is that they do not include Doppler waveforms. Hard copy outputs of these waveforms are usually required for insurance reimbursement if one wishes to bill for services in addition to an office visit. Reporting the quality of waveforms as triphasic, biphasic, monophasic or inaudible can sometimes greatly assist with clinical decision making.
For instance, consider two patients presenting with a normal ABI value of 1.03. The first patient has normal triphasic waveforms but the second patient has barely audible monophasic waveforms. This second patient probably requires further workup like segmental pressures, toe brachial index or referral to primary care for risk factor reduction (smoking, dietary changes, statin drugs, blood sugar control). The second patient may also require referral to a vascular specialist depending on the history and clinical presentation.
Ankle brachial index screening is not appropriate for all patients. Generally, one should avoid screening for patients with active leg cellulitis, severe lymphedema, suspected deep vein thrombosis, severe obesity, overall poor health condition, end-stage renal dialysis, severe dementia and do-not-resuscitate status. I have observed that patients over the age of 90 years old seldom benefit from ABI testing but one must consider the overall clinical picture. Furthermore, ABI screening is not indicated for an asymptomatic 72-year-old male patient who has strong pedal pulses and recently completed a marathon race.
Screening for PAD adds value to your podiatry practice. Patients will appreciate your thoroughness and the extra time you spend with them. Primary care providers will appreciate the test results you send them along with an occasional phone call to discuss a high-risk patient with PAD. Vascular specialists will appreciate your timely referrals and the opportunity to avoid wounds or amputations.
Podiatrists are uniquely poised to help prevent limb loss and should take some time to critically consider using the ABI test to screen for PAD. Will your practice follow evidence-based systematic guidelines like the 2011 ACCF/AHA recommendations or will your practice only look for claudication, the tip of the Titanic-sized PAD iceberg, and ignore what lies underneath the water?
Dr. DeBrule is in private practice with Midwest Podiatry Centers in Richfield, Minnesota. He is board certified in wound care.
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13. Daddato S, Tartagni E, Dormi A, et al. Can peripheral arterial disease be early screened for in a podiatric setting? A preliminary study in a cohort of asymptomatic adults. Eur Rev Med Pharmacol Sci. 2012; 16(12):1646-1650.
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