Draft Recommendation Statement
Peripheral Artery Disease and Cardiovascular Disease: Screening and Risk Assessment With the Ankle-Brachial Index
This opportunity for public comment expired on February 12, 2018 at 8:00 PM EST
Note: This is a Draft Recommendation Statement. This draft is distributed solely for the purpose of receiving public input. It has not been disseminated otherwise by the USPSTF. The final Recommendation Statement will be developed after careful consideration of the feedback received and will include both the Research Plan and Evidence Review as a basis.
Recommendations made by the USPSTF are independent of the U.S. government. They should not be construed as an official position of the Agency for Healthcare Research and Quality or the U.S. Department of Health and Human Services.
Draft: Recommendation Summary
The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of screening for peripheral artery disease (PAD) and cardiovascular disease (CVD) risk with the ankle-brachial index (ABI) in adults.
The U.S. Preventive Services Task Force (USPSTF) makes recommendations about the effectiveness of specific clinical preventive services for patients without obvious related signs or symptoms.
It bases its recommendations on the evidence of both the benefits and harms of the service and an assessment of the balance. The USPSTF does not consider the costs of providing a service in this assessment.
The USPSTF recognizes that clinical decisions involve more considerations than evidence alone. Clinicians should understand the evidence but individualize decision making to the specific patient or situation. Similarly, the USPSTF notes that policy and coverage decisions involve considerations in addition to the evidence of clinical benefits and harms.
PAD is a manifestation of atherosclerosis in the lower limbs. PAD can impair walking and, in severe cases, can lead to tissue loss, infection, and amputation. In addition to morbidity directly caused by PAD, patients with PAD are at increased risk for CVD events, because atherosclerosis is a systemic disease that also causes coronary and cerebrovascular events. The most recent data from the National Health and Nutrition Examination Survey (1999–2004) show that 5.9% of the U.S. population age 40 years or older has a low ABI (≤0.9), which indicates the presence of PAD. The true prevalence of PAD is difficult to establish, because more than half of persons with a low ABI are asymptomatic or have atypical symptoms, and because population screening studies using a gold standard diagnostic test are lacking.
Although the USPSTF found adequate evidence that the ABI is an accurate test for detecting PAD in patients with symptoms, it found few data on the accuracy of the ABI for identifying asymptomatic persons who can benefit from treatment.
Benefits of Early Detection and Intervention or Treatment
The USPSTF found inadequate evidence to assess whether screening for and treatment of PAD in asymptomatic patients leads to clinically important benefits in either preventing the progression of PAD or preventing CVD events.
Harms of Early Detection and Intervention or Treatment
The USPSTF found adequate evidence that the direct harms of screening itself, beyond the time needed for testing, are minimal. Other harms may include false-positive results, exposure to gadolinium or contrast dye if magnetic resonance angiography (MRA) or computed tomography angiography (CTA) is used to confirm a diagnosis of PAD, anxiety, labeling, and opportunity costs. If a low ABI finding prompts further evaluation for CVD, harms could include those attributable to stress testing and angiography. The harms of preventive treatment for PAD or CVD include bleeding (with aspirin use) and possibly diabetes (with statin use).
The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of screening for PAD and CVD risk with the ABI in adults.
Draft: Clinical Considerations
Patient Population Under Consideration
This recommendation applies to asymptomatic adults without a known diagnosis of PAD, CVD, or severe chronic kidney disease.
Suggestions for Practice Regarding the I Statement
In deciding whether to screen for PAD with the ABI in asymptomatic adults, clinicians should consider the following factors.
Potential Preventable Burden
The true prevalence of PAD in the general population is not known. Data from the 1999–2004 National Health and Nutrition Examination Survey show that 5.9% of the U.S. population age 40 years or older (7.1 million adults) has a low ABI (≤0.9), which indicates the presence of PAD.1 The most recent prevalence data available are from a screening trial of Danish men ages 65 to 74 years, which identified a prevalence of 11% when PAD was defined as an ABI less than 0.9 or greater than 1.4. Two-thirds of identified patients reported no intermittent claudication, which is considered the classical symptom of PAD.2
The proportion of asymptomatic persons with a low ABI who will go on to develop symptoms is not known. Studies estimate that among persons with stable claudication but not critical ischemia, approximately 70% to 80% will remain stable over 5 years, while 10% to 20% will have worsening claudication and 1% to 2% will develop critical ischemia, possibly requiring amputation.3 In addition to the risk of worsening symptoms in the lower limbs, a low ABI is associated with increased risk of CVD events. Studies suggest that the 5-year cumulative incidence of cardiovascular mortality is 9% (95% CI, 7 to 12) in asymptomatic patients with a low ABI and 13% (95% CI, 9 to 17) in symptomatic patients with a low ABI; patients with a normal ABI had an average incidence of 5% (95% CI, 4 to 6).4
Although minimal harms are associated with the ABI test itself, downstream harms are possible. False-positive results, false-negative results, anxiety, labeling, and exposure to gadolinium or contrast dye from confirmatory MRA or CTA may occur, while further evaluation of CVD risk may involve stress testing or angiography. If the ABI is used to determine the need for pharmacologic treatment to reduce CVD risk, patients could receive additional treatment with resulting adverse effects or be reclassified to a lower risk category and potentially discontinue treatment that may be beneficial.5
An older study of U.S. primary care practices found that 12% to 13% reported using the ABI for CVD screening weekly or monthly, 6% to 8% reported using it annually, and 68% reported never using it. However, the study was conducted more than a decade ago and may not reflect current practice.6
Assessment of Risk
In addition to older age, major risk factors for PAD include diabetes, current smoking, high blood pressure, high cholesterol level, obesity, and physical inactivity, with current smoking and diabetes showing the strongest association.7 PAD is more common in men than in women but the difference disappears when adjusted for age.5, 8 Among healthy U.S. men ages 40 to 75 years without a history of CVD, the risk for PAD over 25 years in the absence of four conventional cardiovascular risk factors (current smoking, high blood pressure, high cholesterol level, or type 2 diabetes) is rare (9 cases per 100,000 men per year). These four risk factors account for 75% of all cases of PAD, and at least one of these risk factors is present at the time of PAD diagnosis in 96% of men.9
Resting ABI is the most commonly used test for detection of PAD in clinical settings, although variation in measurement protocols may lead to differences in the ABI values obtained. The ABI is calculated as the systolic blood pressure obtained at the ankle divided by the systolic blood pressure obtained at the brachial artery while the patient is lying down. A ratio of less than 1 (typically defined as <0.9) is considered abnormal and is commonly used to define PAD. There are relatively few data on the accuracy of the ABI in asymptomatic populations. One study in men and women older than age 70 years reported that an ABI of less than 0.9 had a sensitivity of 15% to 20% and a specificity of 99% compared to whole-body MRA.10, 11 Physical examination has low sensitivity for detecting mild PAD in asymptomatic persons.5 Although femoral bruit (vascular murmur at the femoral artery), pulse abnormalities, or ischemic skin changes significantly increase the likelihood ratio for a low ABI (≤0.9), these signs indicate moderate to severe obstruction of blood flow or clinical disease.12 The clinical benefits and harms of screening for PAD with a physical examination have not been well evaluated, although it is often performed.5
Treatment and Interventions
Because PAD is a manifestation of systemic atherosclerosis in the lower limbs, treatment of PAD has two potential targets: reducing morbidity and mortality from lower limb ischemia and preventing CVD events due to systemic atherosclerosis. Treatment of PAD itself focuses on improving outcomes in symptomatic patients (e.g., increase walking distance and quality of life by improving symptoms of intermittent claudication and leg function, preventing or reducing limb complications, and preserving limb viability). Interventions to prevent CVD events include smoking cessation, lowering cholesterol level, managing high blood pressure, and antiplatelet therapy. However, since the major risk factors for PAD are also used to calculate CVD risk, patients with a low ABI may already be recommended for these treatments.
Additional Approaches to Prevention
The National Heart, Lung, and Blood Institute provides resources on assessing cardiovascular risk, including a link to an online version of the Pooled Cohort Equations,13 as well as preventing PAD.14 Healthy People 2020 provides a database of evidence-based resources for achieving Healthy People 2020 goals, including interventions to prevent CVD.15
The USPSTF has made recommendations on many factors related to CVD prevention, including screening for high blood pressure,16 statin use,17 screening for diabetes,18 counseling on smoking cessation,19 and counseling on healthful diet and physical activity.20 In addition, the USPSTF recommends low-dose aspirin use in adults at increased risk for CVD.21
Draft: Other Considerations
Research Needs and Gaps
Large population-based, randomized trials of screening versus no screening are needed to determine whether screening for PAD with the ABI improves clinical outcomes. One ongoing study in Denmark has published preliminary results; however, this study limited enrollment to men ages 65 to 74 years and included screening for abdominal aortic aneurysm and high blood pressure, making it difficult to evaluate the benefit of screening with the ABI alone.2 Two other ongoing trials that include the ABI as part of a screening bundle may have the same limitation.22, 23 Future studies, in addition to isolating the effect of individual tests, should address the population of persons at potentially increased risk for PAD who are not already receiving interventions to reduce cardiovascular risk, because this is the population most likely to benefit from an additional screening intervention. Studies of screening with the ABI and interventions to stop disease progression in the lower limbs in populations at high risk (i.e., those with diabetes) would also be valuable.
Burden of Disease
The most recent data (1999–2004) show that 5.9% of U.S. adults age 40 years or older have a low ABI.1 In the United States, a low ABI (typically <0.9) is considered diagnostic for PAD in clinical practice, especially in the presence of symptoms. However, evidence that the ABI is an accurate screening test in asymptomatic adults is limited, so the actual prevalence of PAD is not known. When persons with known coronary artery disease or cerebrovascular disease are excluded, the reported prevalence of PAD in studies is 4.7%.1 Prevalence is higher in older populations; the prevalence of low ABI is 1.9% in adults ages 40 to 59 years, 8.1% in those ages 60 to 74 years, and 17.5% in those age 75 years or older.24 However, the natural history of screen-detected PAD, including the development of morbidity and mortality directly related to atherosclerosis in the lower limbs, is not well known. Therefore, the true burden of asymptomatic PAD is difficult to determine.
Scope of Review
To update its 2013 recommendation, the USPSTF reviewed the evidence on whether screening for PAD with the ABI in generally asymptomatic adults reduces morbidity or mortality from PAD or CVD. The current review expanded on the previous review to include persons with diabetes and interventions that include supervised exercise and physical therapy intended to improve outcomes in the lower limbs.5 The USPSTF also considered in a separate review whether the ABI improves CVD risk prediction when added to current risk assessment models (i.e., Framingham Risk Score and the Pooled Cohort Equations).25
Accuracy of Screening Tests
In practice, a low ABI is used as a surrogate marker for PAD; however, its accuracy as a screening tool for PAD in asymptomatic primary care populations has not been well studied. Only one fair-quality study evaluated the ABI as a screening test compared with a reference standard in a relevant population.11 This study was conducted in Sweden and included 306 participants, all of whom were age 70 years at study entry. The mean interval between the ABI and the reference standard, whole-body MRA, was 16 months. When whole-body MRA showing at least 50% stenosis in the pelvic or lower-limb arteries was used as the reference standard, an ABI of less than 0.9 had a sensitivity of 15% to 20% and a specificity of 99%. Because of its low sensitivity and high specificity, the positive and negative predictive values for the ABI in this study were 82% to 83% and 80% to 84%, respectively, depending on the leg.
Effectiveness of Early Detection and Treatment
The USPSTF found no population-based, randomized trials of the effect of PAD screening. One study, the Viborg Vascular (or VIVA) screening trial, assessed the effects of a screening bundle (screening for PAD, abdominal aortic aneurysm, and high blood pressure), reporting an absolute reduction in mortality of 0.006 (95% CI, 0.001 to 0.011) in the screening arm at 5 years.26 However, the applicability of these results to screening for PAD in the United States is uncertain, given that the contribution of the individual tests was not measured and that screening for high blood pressure is standard care in the United States, as is screening for abdominal aortic aneurysm in selected high-risk populations.
Four studies, two of aspirin therapy and two of exercise therapy, addressed treatment of asymptomatic patients with a low ABI or PAD. Both aspirin studies were large good-quality, randomized, controlled trials (RCTs) that addressed whether asymptomatic men and women with a low ABI (defined as ≤0.95 and ≤0.99, respectively) could benefit from daily aspirin therapy (100 mg/day). One study, Aspirin for Asymptomatic Atherosclerosis (N=3,350), enrolled men and women ages 50 to 75 years and followed them for a mean of 8.2 years.27 The second study, the Prevention of Progression of Arterial Disease and Diabetes (POPADAD) trial (N=1,276), enrolled patients age 40 years or older with diabetes, with a median followup of 6.7 years.28 Neither study showed any significant difference in CVD events or mortality. Both studies reported no significant differences in development of intermittent claudication, and the POPADAD trial found no significant difference in PAD revascularization, bypass surgery, angioplasty, or amputation.
The two exercise studies were a small (N=50) fair-quality U.S. RCT with 12 weeks of followup29 and a good-quality Australian RCT (n=882) with 1 year of followup.30 The intervention in both trials was a combination of risk factor modification and measures to increase physical activity. One-quarter of the participants in the U.S. study and slightly more than half of the participants in the Australian study were asymptomatic. Both studies found no statistically significant differences in their primary outcome of walking distance or secondary outcomes of quality of life or self-reported symptoms, although the U.S. study reported an improvement only in the mean stair climbing component of the Walking Impairment Questionnaire.29, 30 There was no difference in development of PAD symptoms or improvement in quality of life.
Potential Harms of Screening and Treatment
No studies directly addressed the harms of screening for PAD with the ABI. Harms resulting from testing may include false-positive results (about 1%), false-negative results (80% to 85%), exposure to gadolinium or contrast dye from confirmatory MRA or CTA, anxiety, labeling, and opportunity costs.5 The time and resources needed to screen with the ABI in a primary care setting may detract from other prevention activities that may have more benefit.
Two trials addressed the harms of aspirin treatment in asymptomatic persons. The Aspirin for Asymptomatic Atherosclerosis trial reported a nonsignificant trend toward increased major bleeding events requiring hospitalization in the aspirin therapy group compared with the placebo group (hazard ratio, 1.71 [95% CI, 0.99 to 2.97]).30 The POPADAD trial reported a numerical decrease in hemorrhagic cerebrovascular accidents in the aspirin group, but the results were imprecise and not statistically significant.28 The two exercise trials did not report on harms.
Estimate of Magnitude of Net Benefit
The USPSTF found few data on the accuracy of the ABI for identifying asymptomatic persons who can benefit from treatment of PAD or CVD. Studies addressing the benefits of treating screen-detected patients with PAD are sparse; two good-quality studies showed no benefit of using the ABI to manage daily aspirin therapy in unselected populations, and two studies showed no benefit from exercise therapy. No studies addressed the harms of screening, although the potential exists for overdiagnosis, labeling, and opportunity costs. Studies that addressed the harms of treatment showed nonsignificant results. Therefore, the USPSTF concludes that the current evidence is insufficient, and that the balance of benefits and harms of screening for PAD with the ABI in asymptomatic adults cannot be determined.
How Does Evidence Fit With Biological Understanding?
PAD is generally considered to be a manifestation of systemic atherosclerosis. Detection when a patient is asymptomatic may suggest significant atherosclerosis in other vessels, such as the heart or brain, and patients may therefore be at risk for types of CVD other than PAD. Early detection and intervention to reduce atherosclerotic progression and prevent future CVD events could improve health outcomes compared with intervention strategies used in the absence of PAD screening. Patients with minimal or atypical symptoms may limit activity in order to avoid symptoms, leading to further deterioration. In this case, screening and treatment could theoretically prevent deterioration. However, a substantial number of asymptomatic persons with a low ABI may never develop clinical signs or symptoms of CVD or PAD but would still be subjected to the harms of testing and subsequent treatments.
Draft: Recommendations of Others
The American College of Cardiology and the American Heart Association released joint practice guidelines recommending screening with the ABI in patients at increased risk, including: adults age 65 years or older, adults age 50 years or older with risk factors for atherosclerosis or a family history of PAD, and adults younger than age 50 years with diabetes and one other risk factor for atherosclerosis.31 In 2015, the Society for Vascular Surgery recommended against screening with the ABI in adults in the absence of risk factors, history, signs, or symptoms of PAD; however, screening is considered reasonable in adults at higher risk (age >70 years, current smoking, or diabetes; abnormal pulse examination; or other established CVD).32 In 2013, the American Academy of Family Physicians concluded that the current evidence is insufficient to assess the balance of benefits and harms of screening for PAD and CVD risk with the ABI in adults;33 it is currently updating this recommendation.
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Internet Citation: Draft Recommendation Statement: Peripheral Artery Disease and Cardiovascular Disease: Screening and Risk Assessment With the Ankle-Brachial Index . U.S. Preventive Services Task Force. January 2018.