How is Ankle-Brachial Index (ABI) measured in an adult over 50 with risk factors for atherosclerotic disease and suspected Peripheral Arterial Disease (PAD)?

How to Measure the Ankle-Brachial Index (ABI)

Measurement Technique

The ABI is calculated as the ratio of the higher systolic pressure in the ipsilateral dorsalis pedis and posterior tibial arteries divided by the higher of the left and right brachial artery systolic pressures. 1

Step-by-Step Procedure

• Position the patient supine (lying down) and allow them to rest for at least 5-10 minutes before measurement 1

• Measure systolic blood pressure in both brachial arteries using a blood pressure cuff and Doppler device 1

• Use the higher of the two brachial pressures as the denominator for calculating ABI in both legs 1, 2

• Measure systolic pressure at both ankle vessels (dorsalis pedis and posterior tibial arteries) in each leg using a Doppler probe to detect blood flow 1

• For each leg, use the higher of the two ankle pressures (dorsalis pedis or posterior tibial) as the numerator 1, 2

• Calculate ABI separately for each leg: ABI = (Higher ankle pressure) / (Higher brachial pressure) 1, 2

Equipment Required

• Blood pressure cuff (appropriately sized for arm and ankle) 1
• Hand-held Doppler ultrasound device or oscillometric equipment 1, 3
• Sphygmomanometer (manual aneroid or digital) 3

Interpretation of Results

Report ABI results using standardized criteria to allow comparison between patients and over time: 1

• Abnormal: ABI ≤0.90 (confirms PAD diagnosis) 1, 4
• Borderline: ABI 0.91-0.99 (requires further evaluation) 1
• Normal: ABI 1.00-1.40 1, 2
• Noncompressible: ABI >1.40 (suggests arterial calcification) 1

Critical Measurement Pitfalls to Avoid

• Failing to measure both brachial pressures can lead to inaccurate calculations if there is subclavian stenosis or other upper extremity arterial disease 4

• Using the lower ankle pressure instead of the higher one will artificially lower the ABI and may lead to false-positive PAD diagnoses 1, 2

• Not recognizing noncompressible arteries (ABI >1.40) in patients with diabetes or chronic kidney disease, which produces falsely elevated readings despite significant PAD 1

When ABI is Unreliable (>1.40)

In patients with noncompressible tibial arteries, measure the toe-brachial index (TBI) instead: 1

• TBI is calculated as the toe (first digit) systolic pressure divided by the higher brachial artery systolic pressure 1

• A TBI ≤0.70 is abnormal and confirms PAD diagnosis 1, 4

• Digital arteries are rarely affected by medial calcification, making TBI more reliable in diabetes and chronic kidney disease 1

Adjunctive Testing

• Doppler waveforms and plethysmographic pulse volume recordings (PVR) at the ankle can confirm concordance with ABI results and help identify noncompressible arteries 1

• These adjunctive tests are particularly useful when there is discordance between clinical presentation and ABI values 1