How to Perform Ankle-Brachial Index (ABI) in Clinic
The ankle-brachial index (ABI) should be performed using a Doppler ultrasound device following a standardized protocol to ensure accurate diagnosis of peripheral arterial disease (PAD). 1
Patient Preparation
- Position patient supine for 5-10 minutes before measurement
- Ensure patient is relaxed with head and heels fully supported
- Maintain comfortable room temperature (19-22°C/66-72°F)
- Ensure patient has not smoked for at least 2 hours before the test
- Remove any clothing that might restrict access to measurement sites 1
Equipment Required
- 8-10 MHz Doppler ultrasound probe
- Blood pressure cuffs of appropriate sizes (should cover at least 40% of limb circumference)
- Doppler ultrasound gel 1
Step-by-Step Procedure
1. Cuff Placement
- Apply cuffs to both arms
- For ankles, place cuffs 2 cm above the superior aspect of the medial malleolus using parallel (straight) wrapping method 1
- Avoid placing cuffs over wounds, ulcers, or recent bypass grafts 1
2. Brachial Pressure Measurement
- Apply Doppler gel over the brachial artery
- Position Doppler probe at 45-60° angle until clearest signal is heard
- Inflate cuff 20 mmHg above the level where flow signal disappears
- Slowly deflate cuff and record pressure when flow signal reappears
- Measure both arms and use the highest value as the denominator 1
3. Ankle Pressure Measurement
- Locate both posterior tibial (PT) and dorsalis pedis (DP) arteries using Doppler
- Apply gel and position probe at 45-60° angle
- Inflate cuff 20 mmHg above flow signal disappearance
- Slowly deflate and record pressure when flow signal reappears
- Measure both PT and DP pressures in each ankle 1
4. Recommended Measurement Sequence
- Follow a consistent sequence (e.g., right arm → right PT → right DP → left PT → left DP → left arm)
- Repeat measurement of first arm at the end of sequence
- Average the two arm measurements unless they differ by >10 mmHg (then use only second measurement) 1
5. ABI Calculation
- Calculate ABI for each leg separately:
ABI = Highest ankle pressure (PT or DP) / Highest brachial pressure - For more sensitive detection of PAD, use the lower ankle pressure instead of the higher one 2
Interpretation of Results
| ABI Value | Classification | Interpretation |
|---|---|---|
| ≤0.90 | Abnormal | PAD confirmed |
| 0.91-0.99 | Borderline | Possible PAD |
| 1.00-1.40 | Normal | No significant PAD |
| >1.40 | Noncompressible | Medial arterial calcification |
| [3] |
Common Pitfalls and Solutions
- Noncompressible vessels (ABI >1.40): Consider toe-brachial index (TBI) measurement instead 3
- Interarm pressure difference >15 mmHg: Suspect subclavian artery stenosis 1
- Patient unable to lie flat: Consider alternative positioning but note that seated position increases ABI by approximately 0.35 1
- Open wounds or ulcers: Cover with impermeable dressing before cuff placement 1
- Recent bypass graft: Avoid cuff placement over graft due to thrombosis risk 1
- Patient movement: Ensure patient remains still during measurement; consider alternative methods if tremor present 1
Special Considerations
- In diabetic patients, ABI may be less sensitive due to arterial calcification; both ABI ≤0.90 and >1.30 indicate increased cardiovascular risk 3, 4
- Do not use oscillometric blood pressure devices for ABI measurement as they tend to overestimate low pressures and have high failure rates in advanced PAD 1, 5
- When clinical suspicion remains despite normal resting ABI, consider exercise ABI testing 3
By following this standardized protocol, clinicians can obtain reliable ABI measurements for accurate diagnosis of PAD and assessment of cardiovascular risk.