ABPI guide and calculator

What the result means

For diagnosing peripheral arterial disease, the AHA/ACC uses 0.90 or less as abnormal, 0.91 to 0.99 as borderline and above 1.40 as non-compressible; the compression thresholds above come from wound-care guidance (Wounds UK 2022).

How to measure it

Equipment

• Hand-held Doppler with an 8 MHz probe (5 MHz for oedematous or obese legs), ultrasound gel and a manual sphygmomanometer.
• A cuff of the right size for the arm and for the ankle: at least 40% of the limb circumference.

Steps

1. Explain the test. The patient lies flat, as close to supine as they can tolerate, and rests for 10 to 20 minutes. Measure before any compression is applied, and cover ulcers with cling film.
2. Brachial pressures. Put the cuff on the upper arm, find the brachial signal with the probe at 45° pointing towards the heart, inflate until the signal disappears, then deflate slowly. The pressure at which the signal returns is the systolic pressure. Repeat on the other arm and use the higher reading.
3. Ankle pressures. Put the cuff just above the malleoli. Measure the dorsalis pedis and the posterior tibial artery (and the peroneal if either cannot be found) in the same way. Use the higher ankle pressure for that leg.
4. Calculate each leg: highest ankle pressure ÷ highest brachial pressure.
5. Listen to the signal. A triphasic or biphasic signal is normal; a monophasic or damped signal suggests disease even if the index looks normal.

Pitfalls

• An index above 1.3, or one that does not match the clinical picture, is common in diabetes, chronic kidney disease and old age. Use toe pressures or waveforms.
• Do not inflate the cuff over an ulcer, a recent bypass graft or a painful area. Do not measure in a patient with suspected acute DVT.
• A difference of more than 15 to 20 mmHg between the arms suggests subclavian stenosis. Always divide by the higher arm pressure.
• Patients who cannot lie flat because of pain at rest may have critical ischaemia: refer.
• Repeat the ABPI if the ulcer deteriorates or fails to heal or pain increases; otherwise about every 12 months for venous ulcers and every 3 to 6 months for mixed ulcers while compression continues.

At our centres

Every new patient has an ABPI with the duplex scan at the first visit, so you do not need to arrange one before referral. Leg ulcer patients are usually seen in the same week. See referral details.