How do you measure Ankle-Brachial Index (ABI) in a patient, particularly those with a history of diabetes or other conditions that may affect the accuracy of the measurement?

How to Measure Ankle-Brachial Index (ABI)

The ABI should be measured with the patient supine after 5-10 minutes of rest, using a handheld Doppler ultrasound device (8-10 MHz) to measure systolic pressures at both brachial arteries and both ankle arteries (dorsalis pedis and posterior tibial), then calculating the ratio by dividing the higher ankle pressure by the higher brachial pressure for each leg. 1, 2

Patient Preparation

Before beginning the measurement:

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

Equipment Required

• Handheld Doppler ultrasound device (8-10 MHz probe) with Doppler gel 2
• Blood pressure cuffs of appropriate size—cuff width should be at least 40% of limb circumference 2
• Avoid placing cuffs over recent bypass grafts due to thrombosis risk 2

Standardized Measurement Sequence

Follow this specific counterclockwise sequence recommended by the American Heart Association 2:

1. Right brachial artery (first arm measurement)
2. Right posterior tibial artery
3. Right dorsalis pedis artery
4. Left posterior tibial artery
5. Left dorsalis pedis artery
6. Left brachial artery (second arm measurement)

Critical Rule for Arm Pressures

• If the first and second right arm measurements differ by >10 mm Hg, discard the first measurement and repeat the right arm pressure 2
• Otherwise, average the two right arm measurements 2
• Always use the highest brachial pressure from either arm as the denominator 2

Calculation Method

The calculation method depends on your clinical purpose:

For Diagnostic Purposes (Confirming PAD)

ABI = Higher ankle pressure (PT or DP) ÷ Higher brachial pressure 1, 2

• This method provides higher specificity (99% vs 93%) and minimizes overdiagnosis 2
• Use this approach when you need to definitively diagnose PAD 2

For Cardiovascular Risk Assessment

ABI = Lower ankle pressure (PT or DP) ÷ Higher brachial pressure 2

• This method provides higher sensitivity (89% vs 66%) and identifies more at-risk individuals 2
• Use this approach for screening asymptomatic high-risk patients 2

Interpretation of Results

Report ABI values to 2 decimal places using these standardized categories 1:

• ABI ≤0.90: Abnormal—indicates PAD 1
• ABI 0.91-0.99: Borderline—may have PAD, consider exercise ABI if clinical suspicion is high 1
• ABI 1.00-1.40: Normal 1
• ABI >1.40: Non-compressible arteries—indicates arterial calcification 1

Special Considerations for Diabetes and High-Risk Patients

When ABI >1.40 (Non-Compressible Arteries)

This is extremely common in diabetes and chronic kidney disease due to medial arterial calcification (Mönckeberg's sclerosis). 1, 3

In these patients, you must obtain toe pressures with waveforms because:

• Arterial calcification makes vessels stiff and resistant to compression 3
• This artificially elevates pressure readings and masks true PAD 3
• Toe-brachial index (TBI) ≥0.75 reliably excludes PAD in diabetic patients 3
• TBI <0.70 indicates PAD 3
• Toe pressure <30 mmHg indicates critical limb-threatening ischemia requiring urgent vascular evaluation 3

Diabetic Foot Infections

When assessing patients with diabetic foot infections, measure ABI using sphygmomanometers and handheld Doppler to assess for peripheral arterial disease, especially if pedal pulses are absent or diminished 1

The interpretation for diabetic patients follows these thresholds 1:

• >1.30: Poorly compressible vessels, arterial calcification
• 0.90-1.30: Normal
• 0.60-0.89: Mild arterial obstruction
• 0.40-0.59: Moderate obstruction
• <0.40: Severe obstruction

Common Pitfalls and How to Avoid Them

Pitfall 1: Using Automated Oscillometric Devices

Do not use automated blood pressure devices for ABI measurement in diabetic or vascular patients. Research shows these devices:

• Systematically overestimate ankle pressures in patients with reduced pressures 4
• Have only 71% sensitivity for detecting ABI ≤0.9 4
• Cannot replace handheld Doppler in diabetes patients 5
• Show poor inter-observer reproducibility (R-coefficient 0.44) 6

Pitfall 2: Relying on Pulse Palpation

Never use pulse palpation alone to measure ABI—it has poor intra-observer reproducibility (R-coefficient 0.60) and significantly underestimates ABI compared to Doppler (mean 0.85 vs 1.03) 6

Pitfall 3: Single Measurements in Borderline Cases

When ABI is borderline (0.91-0.99), request repeat measurements for confirmation rather than relying on a single value 2

Pitfall 4: Missing PAD in "Normal" ABI

If clinical suspicion remains high despite normal resting ABI:

• Consider post-exercise ABI—some patients have normal resting ABI but abnormal values after exercise 7, 2
• In diabetic patients with ABI 0.91-1.40, arterial calcification may be "normalizing" a truly abnormal value 3
• Obtain toe pressures to definitively exclude PAD 3

Pitfall 5: Not Measuring Both Legs

Always measure ABI in both limbs—PAD does not progress symmetrically 7

Training Requirements

Personnel performing ABI should have 2:

• Basic knowledge of vascular anatomy and physiology
• Understanding of clinical presentation of PAD
• Knowledge of how Doppler devices function
• Demonstrated ability to produce reproducible results in both healthy individuals and those with PAD
• Both didactic and experiential training with emphasis on correct technique