What is a normal Ankle-Brachial Index (ABI), how is it performed, and what are its clinical correlations?

Ankle-Brachial Index (ABI): Normal Values, Procedure, and Clinical Correlations

Normal ABI Values

A normal ABI ranges from 0.91 to 1.40, with the optimal range being 1.11 to 1.40, indicating adequate arterial blood flow to the lower extremities and lower cardiovascular risk. 1, 2

• Values 0.91-1.00 are considered "borderline" and warrant further clinical evaluation, as these patients may have increased cardiovascular risk despite not meeting criteria for peripheral artery disease (PAD) 1
• ABI ≤0.90 indicates PAD and confirms the diagnosis in symptomatic patients 1
• ABI >1.40 suggests non-compressible arteries due to medial arterial calcification (Mönckeberg's sclerosis), commonly seen in diabetes and chronic kidney disease 1, 2

The relationship between ABI and mortality follows a reverse J-shaped curve, with the lowest risk occurring in the 1.11-1.40 range and increased risk at both low (<0.90) and high (>1.40) values 2.

How ABI is Performed

Patient Preparation

• Position the patient supine with head and heels fully supported for 5-10 minutes before measurement in a room temperature of 19°C-22°C (66°F-72°F) 3
• Ensure the patient has not smoked for at least 2 hours prior to measurement, as smoking decreases ankle pressures 3
• Cover any open wounds with impermeable dressing 3

Equipment Required

• 8-10 MHz Doppler ultrasound probe with Doppler gel 3
• Blood pressure cuffs of appropriate size (width should be at least 40% of limb circumference) 3

Measurement Sequence

The standardized sequence is critical for accuracy: 3

1. First arm (brachial artery)
2. First posterior tibial (PT) artery
3. First dorsalis pedis (DP) artery
4. Other PT artery
5. Other DP artery
6. Other arm (brachial artery)

If the first arm's systolic blood pressure exceeds the second arm by >10 mm Hg, repeat the first arm measurement and disregard the initial reading. 3

Calculation Methods

The ABI is calculated as the ratio of ankle systolic pressure to brachial systolic pressure, but the specific method depends on the clinical purpose: 1, 3

• For diagnostic purposes (confirming PAD): Use the higher ankle pressure (PT or DP) divided by the higher brachial pressure—this improves specificity (0.99 vs 0.93) and minimizes overdiagnosis 3, 4
• For cardiovascular risk assessment: Use the lower ankle pressure divided by the higher brachial pressure—this identifies more at-risk individuals (sensitivity 0.89 vs 0.66) 3, 4

For each patient, use the lower ABI from both legs for clinical decision-making, as PAD may not progress symmetrically 4.

Interpretation and Clinical Correlations

Diagnostic Performance

• Sensitivity: 69-79% (lower in diabetes: 53-70%) 1
• Specificity: 83-99% compared with angiography showing significant stenoses 1
• Area under ROC curve: 0.87-0.95 for Doppler-measured ABI 1

Clinical Correlations by ABI Value

ABI <0.90 (PAD Present):

• Confirms diagnosis of lower-extremity PAD 1
• Independent predictor of cardiovascular mortality, more important for survival than clinical history of coronary artery disease 5
• Inverse correlation with non-fatal cardiac events (myocardial infarction, stroke, heart failure) 5
• Patients with very low ABI (<0.3) have significantly higher additional cardiovascular risk 5

ABI 0.91-1.00 (Borderline):

• Increased cardiovascular risk compared to higher normal values 1
• Further evaluation is appropriate, including consideration of post-exercise ABI or imaging 1

ABI 1.11-1.40 (Optimal Normal):

• Lowest cardiovascular risk range 2
• Adequate arterial blood flow to lower extremities 2

ABI >1.40 (Non-compressible Arteries):

• Increased risk of cardiovascular events and mortality 1, 2
• Indicates medial arterial calcification, making arteries stiff and resistant to compression 2
• Calcified arteries often coexist with occlusive PAD, masking the true diagnosis 2
• When ABI >1.40 with clinical suspicion of PAD, perform toe-brachial index (TBI) or other imaging studies 1

Cardiovascular Risk Stratification

Individuals with ABI <0.90 or >1.40 should be considered at increased risk of cardiovascular events and mortality, independent of symptoms or other risk factors. 1

The ABI provides incremental information beyond standard risk scores in predicting future cardiovascular events 1.

Common Pitfalls and How to Avoid Them

When ABI is Normal but Clinical Suspicion Remains

If resting ABI is >0.90 but clinical suspicion of PAD persists, perform post-exercise ABI or imaging. 1

• Post-exercise ankle pressure decrease >30 mm Hg or ABI decrease >20% is diagnostic for PAD 1
• ABI may be normal at rest but abnormal after exercise in some PAD patients 2

In Patients with Diabetes or Chronic Kidney Disease

ABI has reduced sensitivity in diabetes and end-stage renal disease due to arterial calcification. 1, 3

• When ABI >1.40, use TBI (normal >0.70, abnormal ≤0.70) as an alternative diagnostic test 1, 3
• Toe vessels are less susceptible to calcification than ankle vessels 1

Measurement Errors to Avoid

• Not measuring both arms and both ankle vessels could miss unilateral disease 6
• Relying on a single measurement when values are borderline (0.91-1.00)—repeat measurements improve accuracy 1, 2
• Using incorrect cuff size—width should be at least 40% of limb circumference 3
• Placing cuff over recent bypass grafts risks thrombosis 3

Calculation Method Confusion

Using the higher ankle pressure for diagnostic purposes prevents overdiagnosis, but using the lower ankle pressure for risk assessment identifies more at-risk patients. 3, 4

A study demonstrated that when only the higher ankle pressure is used, a group of patients at high risk for cardiovascular events (10.8% of the population) is overlooked, with these patients having comparable event rates to those with confirmed PAD (25.0% vs 28.4%) 4.

Compression Therapy Considerations

Always perform ABI before applying compression therapy to identify patients with PAD who may be at risk for complications. 2

• ABI <0.50 is an absolute contraindication for compression therapy due to high risk of tissue damage 2
• ABI 0.50-0.80 requires modified compression and careful monitoring 2
• Failing to perform ABI before compression in patients with lower extremity wounds can lead to tissue necrosis and limb loss 2

Training Requirements

Technicians performing ABI must have basic knowledge of vascular anatomy, physiology, and PAD presentation, plus understanding of Doppler device function. 1, 3

Trainees should demonstrate reproducible results in both healthy individuals and those with PAD, with correct calculation and interpretation of normal and abnormal values 1.