When to Order Ankle-Brachial Index
Order a resting ABI in patients aged 65 years or older, OR in those aged 50 years or older with a history of smoking or diabetes, OR in any patient with exertional leg symptoms or nonhealing wounds. 1
Primary Indications for ABI Testing
The ACC/AHA guidelines provide clear Class I recommendations for when ABI should be ordered 1:
Age-Based Screening
- All patients ≥65 years old should be considered for ABI testing, based on epidemiologic data showing 21% prevalence of PAD (symptomatic or asymptomatic) in this age group 1
- Patients aged 50-64 years with cardiovascular risk factors (smoking or diabetes) warrant ABI measurement 1
Symptom-Based Testing
- Exertional leg symptoms (claudication) - any age 1
- Nonhealing wounds of the lower extremities - any age 1
Risk-Based Screening
In patients at increased cardiovascular risk but without symptoms, ABI measurement is reasonable (Class IIa recommendation) 2. This includes patients with:
- Known atherosclerotic disease in other vascular beds (coronary, carotid, renal) 2
- Multiple cardiovascular risk factors 2
Baseline Confirmation Testing
Once PAD is suspected or diagnosed, measure ABI in both legs to confirm the diagnosis and establish a baseline for future comparison 1. This bilateral measurement is essential regardless of which leg is symptomatic 1.
When NOT to Order ABI
Do not order ABI in patients without increased PAD risk and no suggestive symptoms (Class III recommendation) 2. The U.S. Preventive Services Task Force found insufficient evidence to support screening asymptomatic adults without known cardiovascular disease or diabetes 3.
Important Caveats and Alternative Testing
Noncompressible Vessels
When ABI >1.40 (noncompressible vessels), order a toe-brachial index (TBI) instead 1. This situation occurs commonly in:
The TBI can be obtained in 82% of limbs where ABI is unobtainable and should be considered abnormal when ≤0.6 5.
Normal Resting ABI with Symptoms
In patients with exertional leg symptoms but normal resting ABI (0.91-1.40), order exercise treadmill ABI testing 2. Studies show that 31% of symptomatic patients with normal resting ABI will have abnormal post-exercise ABI, revealing hemodynamically significant PAD 6.
Clinical Reasoning
The 2011 ACC/AHA focused update lowered the screening age from 70 to 65 years based on the German Epidemiologic Trial, which demonstrated high PAD prevalence in this population 1. This change reflects the intent to identify PAD earlier in at-risk populations, as PAD is a strong predictor of cardiovascular morbidity and mortality 7.
The ABI serves dual purposes: diagnosing lower extremity PAD and identifying patients at increased cardiovascular risk who may benefit from aggressive risk factor modification 8, 9. An abnormal ABI (≤0.90) is associated with 2-3 times higher risk of cardiovascular events, independent of traditional risk factors 7.
Common Pitfalls to Avoid
- Don't rely on physical examination alone - it has low sensitivity for detecting mild PAD in asymptomatic patients 3
- Don't stop at a normal resting ABI in symptomatic patients - nearly half of patients referred for suspected PAD have normal resting ABI and require exercise testing 6
- Don't use ABI in patients with noncompressible vessels - use TBI instead to avoid false-normal results 1, 4
- Don't screen low-risk asymptomatic patients - this leads to overdiagnosis without proven benefit 3