How is the ankle‑brachial index measured, interpreted, and managed?

Ankle-Brachial Index: Measurement, Interpretation, and Management

The ankle-brachial index (ABI) should be measured using a handheld Doppler device with the higher of the dorsalis pedis or posterior tibial artery pressure divided by the higher brachial artery pressure, with an ABI <0.90 confirming peripheral arterial disease (PAD) and values <0.90 or >1.40 indicating significantly increased cardiovascular risk. 1

How to Measure the ABI

Equipment and Technique

• Use a handheld Doppler device with appropriate gel applied over the sensor
• Position the probe at 45° to 60° angle to the skin surface
• Move the probe until the clearest signal is heard 1

Measurement Sequence

Follow this specific order consistently 1:

1. First arm (brachial)
2. First leg posterior tibial (PT) artery
3. First leg dorsalis pedis (DP) artery
4. Second leg PT artery
5. Second leg DP artery
6. Second arm (brachial)
7. Repeat the first arm measurement

Blood Pressure Measurement Protocol

• Inflate the cuff progressively to 20 mm Hg above flow signal disappearance
• Maximum inflation is 300 mm Hg
• Deflate slowly to detect pressure at flow signal reappearance 1

Critical Measurement Rule

If the difference between the two first arm measurements exceeds 10 mm Hg, discard the first measurement and use only the second measurement 1. Otherwise, average the two first arm measurements to account for white coat effect.

How to Calculate the ABI

The Calculation Formula

ABI = Higher ankle pressure (PT or DP) ÷ Higher brachial pressure (left or right arm) 2

Which Ankle Pressure to Use: A Critical Decision

For diagnostic purposes (confirming suspected PAD): Use the higher of the PT or DP pressures at each ankle 1

• This provides higher specificity (99% vs 93%)
• Minimizes overdiagnosis and unnecessary testing
• Prevents false positives in healthy subjects

For cardiovascular risk stratification: Use the lower of the PT or DP pressures 1, 3

• Identifies more patients at cardiovascular risk
• Better captures unilateral or asymmetric disease
• The lower ABI identifies individuals who would be missed using the higher value

Important caveat: If one ankle artery signal is absent but the other ankle artery shows normal ABI, perform duplex ultrasound to rule out PAD 1

Interpretation of ABI Values

Diagnostic Thresholds 1

ABI Value	Interpretation	Action Required
<0.90	PAD confirmed	Diagnose PAD; assess cardiovascular risk
0.91-1.00	Borderline	Further evaluation appropriate; consider exercise ABI
1.00-1.40	Normal	No PAD
>1.40	Non-compressible vessels	Use toe-brachial index (TBI) or other tests

When ABI is Unreliable

ABI >1.40 indicates vessel calcification (common in diabetes and chronic kidney disease), making the test unreliable 2. In these cases:

• Obtain toe-brachial index (TBI) with waveforms
• Normal TBI is >0.70 2
• Consider transcutaneous oxygen pressure (TcPO2) or skin perfusion pressure (SPP)

Exercise ABI Testing

Indications 1

Perform exercise ABI when:

• Resting ABI is >0.90 but clinical suspicion for PAD remains high
• Patient has exertional leg symptoms suggestive of claudication

Diagnostic Criteria for Abnormal Exercise ABI 1

• Post-exercise ankle pressure decrease >30 mm Hg, OR
• Post-exercise ABI decrease >20%

Management Based on ABI Results

For Asymptomatic Patients with Abnormal ABI

ABI <0.90 or >1.40 = High cardiovascular risk 1

• Hazard ratio for cardiovascular mortality: 4.2 in men, 3.5 in women 1
• Initiate aggressive cardiovascular risk factor modification
• Consider these patients equivalent to coronary artery disease patients for risk stratification

For Symptomatic Patients

ABI <0.90 with symptoms: 1

• Confirm PAD diagnosis
• If symptoms are functionally limiting, obtain anatomic imaging (duplex ultrasound, CTA, MRA, or catheter angiography) to plan revascularization 2
• Initiate guideline-directed medical therapy and structured exercise program

ABI >0.90 with persistent symptoms:

• Perform exercise ABI testing
• If exercise ABI is abnormal, proceed with anatomic imaging
• If exercise ABI is normal, seek alternative diagnosis 2

Common Pitfalls and How to Avoid Them

Measurement Errors

Problem: Studies show ABI is often incorrectly performed in primary care, with great variability compared to vascular laboratory measurements 4

Solutions:

• Use standardized technique consistently
• Ensure proper training for personnel performing the test 1
• When ABI values are close to threshold (0.80-1.00), consider repeating or referring to vascular laboratory

Automated Devices

Automated oscillometric devices have poor concordance with Doppler 5

• Sensitivity as low as 8-33% compared to Doppler
• Do not use automated devices for PAD screening in primary care
• Stick with handheld Doppler as the standard 5

Reproducibility Concerns

Reproducibility is only crucial when ABI is near threshold values 1

• With 95% CI of ±0.10, an ABI ≤0.80 is definitively abnormal
• An ABI ≥1.00 is definitively normal
• Values between 0.80-1.00 may warrant repeat measurement

Cardiovascular Risk Prediction

The ABI provides incremental prognostic information beyond traditional risk scores 1. For every patient with ABI <0.90:

• Intensify cardiovascular risk factor management
• Screen for coronary and carotid artery disease
• Consider as high-risk for cardiovascular events regardless of symptoms

The ABI remains one of the most cost-effective tests providing both diagnostic accuracy for PAD and robust prognostic information for cardiovascular events 6, 7.