Controversy persists over the ABI test and how to read its results. The author sorts through the evidence to arrive at some recommendations.
The ankle-brachial index (ABI) is an important noninvasive office test with proven ability to detect peripheral arterial disease (PAD) and predict cardiovascular risk. The ABI is broadly accepted as a screening tool for patients with traditional symptoms of claudication, but there is disagreement over its appropriateness for asymptomatic patients. Clearly there is evidence on both sides of the debate. Read on to see if you and I come down on the same side of the matter.
About 8 million Americans are thought to have PAD, an atherosclerotic disease of the peripheral vasculature. The most common symptom is chronic leg pain, or intermittent claudication, but 50%-75% of PAD patients are asymptomatic. PAD is a serious disease that can lead to limb ischemia and subsequent amputation. Additionally, patients are at increased risk for cardiovascular morbidity and mortality.
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Interpreting the findings
A normal ABI is in the range of 1.1-1.3. When verified by angiography, an ABI <0.9 has a sensitivity and specificity of 95% for PAD.1 Although there is some controversy concerning the cutoff levels, the ABI can also be used as an indicator of PAD severity. In general, an ABI <0.9 is considered abnormal, and traditional symptoms of claudication are generally seen at 1.3, may also indicate disease.
Unfortunately, the test can be insensitive in patients with diabetes who tend to have more calcified, less compressible vessels. The alternative to an ABI in these patients is a toe-brachial index (TBI), which can be done with a referral at most vascular labs.
Why it should be used
The PAD Awareness, Risk, and Treatment: New Resources for Survival (PARTNERS) study followed 6,979 high-risk patients (older than 70 years or aged 50-69 with a history of smoking or diabetes). In this study, only 11% of those with PAD were found to have traditional symptoms of intermittent claudication.2 Therefore, limiting ABI to patients with traditional symptoms could miss almost 90% of those with the disease.
The ABI is not only a valuable screening tool for PAD, it has also been shown to predict cardiovascular events such as MI and stroke. A large study in Europe, the German Epidemiological Trial on Ankle-Brachial Index (getABI), tracked 6,880 patients in a primary-care setting for three years. The researchers found that compared with participants who had an ABI >1.1, those with an ABI <0.5 had five times greater risk for cardiovascular events, such as MI, ischemic stroke, carotid revascularization, coronary revascularization, critical leg ischemia, or amputation. After adjustment for other, recognized cardiovascular disease (CVD) risk factors (smoking, diabetes, BP, blood lipid levels), those with PAD sustained almost four times the CVD risk as those without PAD.3 A meta-analysis of studies tracking ABI and cardiovascular events found that the ABI had a sensitivity and specificity of 16.5% and 92.7% for coronary heart disease. The comparable figures for stroke were 16.0% and 92.2%, respectively.4
A low ABI has been shown consistently to correlate with increased risk for cardiovascular events. This holds true even after adjusting for traditional risk factors. The Edinburgh Artery Study tracked 1,592 patients aged 55-74 for 12 years. They found that an ABI <0.9 was associated with a statistically significant increase in risk when used in addition to a traditional risk-prediction model. After calculating risk for fatal MI, the addition of a low ABI carried the same risk as any one of the following: high systolic BP, low HDL, high total cholesterol, or history of smoking.5
Special populations
As with CVD, evidence indicates that PAD becomes more prevalent with age. Data obtained through the National Health and Nutrition Examination Survey (NHANES) show that 4.3% of people older than 40 had a diagnosis of PAD. That figure tripled to 14.5% among those older than 70.6 There was no difference in prevalence by gender, but other studies have contradicted that finding.
Additional risk factors for PAD are also similar to those for heart disease. NHANES data showed that “traditional” risk factors for CVD can also be useful for predicting PAD, i.e., hypertension, high cholesterol, diabetes, and smoking. NHANES found that 95% of persons with PAD had at least one of these risk factors and that 72% had two or more. Another study found that low kidney function and African-American ethnicity were also risks.5
Another study, which tracked ABIs and TBIs over 10 years, found diabetes to be a significant predictor of a decreased TBI but not of a decreased ABI. This is further evidence to support the use of TBI in diabetic patients. The independent predictors of a decrease in ABI to <0.3 were current smoking and a high ratio of HDL-to-total cholesterol.7
Recommendations
Experts suggest that the ABI be used in conjunction with the Framingham scale to further stratify cardiovascular risk in intermediate risk patients older than 60 years of age.8 They propose that an abnormal ABI in these patients might increase their cardiovascular risk category. Others recommend that the ABI be used in people from 50-70 years of age with at least one recognized cardiovascular risk factor, such as smoking, low HDL, high total cholesterol, or hypertension.4
The American Diabetes Association suggests that an ABI be obtained on all people with diabetes older than 50. However, as mentioned previously, a more appropriate test for those with diabetes may be the TBI.
In contrast to these recommendations, the U.S. Preventive Services Task Force (USPSTF) advises against ABI screening for asymptomatic PAD. The Task Force found only “fair evidence” that the ABI can detect PAD in asymptomatic individuals. However, they gave the practice a D rating (can potentially cause harm) because of the low prevalence of PAD in the general population, the perceived lack of treatment options, and the potential for unnecessary workups and false positives, which could lead to unnecessary personal hardship and health-care expenditure.9 The AHA has asked the Task Force to reconsider its recommendation.
Roadblocks to widespread use of ABIsIn addition to lack of endorsement by the USPSTF for ABI screening in asymptomatic patients, there are other barriers to routine use of the ABI. Perhaps the most significant is the lack of reimbursement. Generally, Medicare will not reimburse for ABIs used to screen asymptomatic patients. Patients must either have a history of vascular disease or show the signs and symptoms of limb ischemia. Moreover, even when a patient qualifies, a standard office reading does not suffice for billing purposes. Most insurance companies require that the ABI be obtained by using a Doppler device that produces a printout of the test. This technology is usually available only at vascular labs.
Reimbursement is not the only barrier to broader use of the ABI. There is also uncertainty over how to interpret results. While it is generally accepted that an index 1.3 and those from 0.9-1.1. One study recently found similar cardiovascular event rates in those with high ABIs and normal ABIs.10 However, ABIs >1.5 are often thought to be a result of noncompressible veins and are commonly excluded from studies.
Conclusion
There is broad agreement that primary-care providers should be using the ABI to assess patients presenting with classic PAD symptoms. But as the USPSTF suggests, using the ABI as a populationwide screening tool, without regard to age and other risk factors, would be counterproductive. The effort expended in obtaining universal ABIs would return a low yield and would usually be nonreimbursable by insurance companies.
On the other hand, the ABI should be recognized as a potentially valuable tool for cardiovascular risk stratification and detection of asymptomatic limb ischemia.
Consider obtaining an ABI in patients presenting with atypical leg pains and cardiovascular risk factors. An ABI should also be considered in the following patients older than age 50: those at intermediate risk for cardiovascular events as determined by the Framingham scale, those with diabetes, those with renal insufficiency, and those of African-American descent with one of the above risk factors. By obtaining an ABI in high-risk individuals—but not restricting its use to those with symptomatic PAD—clinicians can best utilize this valuable screening tool.
Ms. Kirner is a recent graduate of Pacific Lutheran University in Tacoma, Wash., where she trained to be a family nurse practitioner.
References
1. U.S. Preventive Services Task Force. Screening for peripheral arterial disease: recommendation statement. Am Fam Physician. 2006;73:497-500.
2. Hirsch AT, Criqui MH, Treat-Jacobson D, et al. Peripheral arterial disease, detection, awareness, and treatment in primary care. JAMA. 2001;286:1317-1324.
3. Diehm C, Lange S, Darius H, et al. Association of low ankle-brachial index with high mortality in primary care. Eur Heart J. 2006;27:1743-1749.
4. Doobay AV, Anand SS. Sensitivity and specificity of the ankle-brachial index to predict future cardiovascular outcomes: a systematic review. Arterioscler Thromb Vasc Biol. 2005;25:1463-1469.
5. Lee AJ, Price JF, Russell MJ, et al. Improved prediction of fatal myocardial infarction using the ankle brachial index in addition to conventional risk factors: The Edinburgh Artery Study. Circulation. 2004;110:3075-3080.
6. Selvin E, Erlinger TP. Prevalence of and risk factors for peripheral arterial disease in the United States: results from the National Health and Nutrition Examination Survey, 1999–2000. Circulation. 2004;110:738-743.
7. Aboyans V, Criqui MH, Denenberg JO, et al. Risk factors for progression of peripheral arterial disease in large and small vessels. Circulation. 2006;113: 2623-2629.
8. Greenland P, Smith SC Jr, Grundy SM. Improving coronary heart disease risk assessment in asymptomatic people: role of traditional risk factors and noninvasive cardiovascular tests. Circulation. 2001;104:1863-1867.
9. U.S. Preventive Services Task Force. Screening for peripheral arterial disease. Available at www.ahrq.gov/clinic/uspstf/uspspard.htm. Accessed July 23, 2007.
10. Wattanakit K, Folsom AR, Duprez DA, et al. Clinical significance of high ankle-brachial index: insights from the Atherosclerosis Risk in Communities (ARIC) study. Atherosclerosis. 2007;190:459-464.
