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