What is the ankle‑brachial index (ABI) and how is it used in the diagnosis and management of peripheral arterial disease (PAD)?

Ankle-Brachial Index (ABI) in Peripheral Arterial Disease

The ABI is the first-line, noninvasive diagnostic test for PAD and should be measured in all patients with suspected lower extremity PAD—defined as those with exertional leg symptoms, nonhealing wounds, age ≥65 years, or age ≥50 years with smoking or diabetes history. 1

What is the ABI?

The ABI is the ratio of systolic ankle arterial pressure to systolic brachial arterial pressure. The higher of the two brachial pressures (right or left) serves as the denominator, and the higher of the two ankle pressures (posterior tibial or dorsalis pedis) serves as the numerator for each leg. 2 The test is performed using a blood pressure cuff and continuous-wave Doppler device while the patient is supine 3.

Diagnostic Interpretation

ABI results should be uniformly reported as: 1

• Noncompressible: >1.40
• Normal: 1.00-1.40
• Borderline: 0.91-0.99
• Abnormal (PAD confirmed): ≤0.90

An ABI <0.90 has 69-79% sensitivity and 83-99% specificity for detecting significant arterial stenoses 4, making it highly reliable for PAD diagnosis.

Who Should Be Tested?

Mandatory Testing (Class I Recommendations):

• Symptomatic patients: Those with exertional leg symptoms or nonhealing wounds 1
• Age-based screening: All individuals ≥65 years 1
• High-risk younger patients: Age ≥50 years with smoking history or diabetes 1
• All patients with diabetes and foot ulcers 5

Reasonable Screening (Class IIa):

• Asymptomatic adults >50 years with PAD risk factors (smoking, diabetes, hypertension, hypercholesterolemia) 3

Not Recommended:

• Patients <50 years without risk factors or symptoms (prevalence ~1%, low yield) 4

Critical Limitations and When ABI Fails

The ABI becomes unreliable when >1.40 due to noncompressible, calcified arteries—common in patients with diabetes, chronic kidney disease, and advanced age 3. In these cases:

Alternative tests must be used: 1, 4

• Toe-Brachial Index (TBI): Use cutoff <0.70 (some sources use <0.60)
• Doppler waveforms or pulse volume recordings (PVR)
• Transcutaneous oxygen pressure (TcPO₂)

Exercise ABI Testing

When resting ABI is 0.91-1.40 but clinical suspicion remains high, perform exercise treadmill ABI testing. 6, 4 A postexercise ankle pressure decrease >30 mmHg or ABI decrease >20% confirms PAD 6.

Prognostic Value Beyond Diagnosis

ABI predicts cardiovascular morbidity and mortality independent of symptoms. 2 PAD is a coronary heart disease risk equivalent, and an abnormal ABI (<0.90 or >1.40) identifies patients at increased risk for myocardial infarction, stroke, and death 2, 6. This changes management of risk factors including hypertension and dyslipidemia.

Borderline ABI values (0.91-1.00) also warrant further cardiovascular risk evaluation 6, as these patients occupy an intermediate risk category.

Management Implications in Diabetic Foot Ulcers

For patients with diabetes and foot ulcers, ABI measurement is mandatory as part of initial evaluation 5. Specific thresholds guide urgent intervention:

Consider urgent vascular imaging and revascularization when: 5

• ABI <0.5 or ankle pressure <50 mmHg
• Toe pressure <30 mmHg
• TcPO₂ <25 mmHg

Consider revascularization when ulcers fail to improve within 6 weeks despite optimal management, regardless of bedside test results 5.

Common Pitfalls to Avoid

1. Using only one ankle artery: Always measure both dorsalis pedis and posterior tibial pressures—using only one may miss significant disease 2
2. Ignoring arm pressure discrepancy: If brachial pressures differ by >10 mmHg, remeasure the first arm 6
3. Accepting falsely normal ABI in diabetes: Always consider TBI or alternative testing when ABI >1.40 3
4. Screening low-risk young patients: Avoid routine testing in those <50 years without risk factors 4
5. Missing intra-leg asymmetry: Significant differences between DP- and PT-derived ABI values (≥0.15) may predict worse limb outcomes 7

Measurement Sequence

Adopt this standardized sequence: 6

1. First arm
2. First posterior tibial artery
3. First dorsalis pedis artery
4. Other posterior tibial artery
5. Other dorsalis pedis artery
6. Other arm

Performance Measures

ABI should be documented at least once every 5 years in patients with cardiovascular disease 2, though this represents a performance measure rather than a clinical recommendation for individual patient care.

The ABI remains underutilized despite its diagnostic accuracy, prognostic value, and low cost 8, 9. When performed correctly with attention to its limitations, it provides essential information for both PAD diagnosis and cardiovascular risk stratification.