Ankle brachial index and toe pressures — indications, assessment and interpretation

Definition and principle. The ankle brachial index (ABI) is the ratio of the highest systolic pressure at the ankle to the highest systolic pressure in either arm, measured with a sphygmomanometer and a hand-held Doppler probe. In health the ankle pressure equals or slightly exceeds the brachial pressure because of pulse-wave amplification; a proximal arterial stenosis causes a pressure drop across the lesion, so the ABI falls. It is a simple, non-invasive, inexpensive and reproducible bedside test and is the first-line investigation for peripheral arterial disease (PAD).

Indications. Diagnostic: symptoms of PAD — intermittent claudication, rest pain, non-healing foot ulcer or gangrene; absent or diminished pulses or a femoral bruit. Before compression therapy: mandatory before applying compression bandaging or stockings for a leg ulcer, because compression in the presence of significant arterial disease causes ischaemic necrosis. Screening in asymptomatic high-risk groups: age ≥ 65 (or ≥ 50 with diabetes or smoking), diabetes with a foot at risk, chronic kidney disease, known atherosclerosis elsewhere (coronary, carotid, aortic aneurysm). Prognostic: as a marker of generalised atherosclerosis and cardiovascular risk. Monitoring: after revascularisation and in follow-up of known PAD.

How to perform it. Rest the patient supine for 5–10 minutes in a warm room. Use an appropriately sized cuff and an 8–10 MHz continuous-wave Doppler probe. Measure the brachial systolic pressure in both arms and take the higher. At each ankle, measure the pressure over the dorsalis pedis and the posterior tibial arteries and take the higher of the two. ABI for each leg = highest ankle pressure in that leg ÷ highest brachial pressure of the two arms. Calculate each leg separately and report the lower of the two values as the patient's ABI. Do not use a stethoscope — Korotkoff sounds are unreliable at the ankle. If claudication is suspected but the resting ABI is normal, perform an exercise ABI (treadmill or heel raises): a post-exercise fall of more than 20%, or an absolute ankle pressure drop of more than 30 mmHg, is diagnostic.

Interpretation. ABI value Interpretation > 1.40 Non-compressible, calcified vessels — the result is uninterpretable; proceed to toe pressures 1.00 – 1.40 Normal 0.91 – 0.99 Borderline — perform an exercise ABI ≤ 0.90 Diagnostic of peripheral arterial disease (sensitivity ~75–90%, specificity ~95% against angiography) 0.40 – 0.90 Mild to moderate PAD — usually claudication < 0.40 Severe PAD / chronic limb-threatening ischaemia — rest pain, ulceration, gangrene An ABI of 0.90 or less roughly doubles 10-year mortality and major coronary event rates, so an abnormal ABI is an indication for full cardiovascular secondary prevention regardless of leg symptoms.

Limitations — the crucial geriatric caveat. The ABI is falsely NORMAL or falsely HIGH when the tibial arteries are rigid and non-compressible due to medial (Mönckeberg) calcification — classically in long-standing diabetes, chronic kidney disease and dialysis, and advanced age. This is precisely the population most likely to have severe disease, so a normal ABI in a diabetic patient with an ischaemic-looking foot must never be accepted at face value. Other limitations: it gives no anatomical localisation, it can miss isolated iliac (proximal) stenosis if collaterals are good, it is inaccurate with leg ulceration, oedema, pain or amputation, and it is operator-dependent.

Toe pressures and the toe brachial index (TBI). The digital arteries are largely spared from medial calcification, so toe pressures remain reliable when the ABI does not. Measure with a small (2–2.5 cm) cuff on the great toe and photoplethysmography or laser Doppler, after warming the foot. Normally the toe pressure is about 30 mmHg lower than the ankle pressure. TBI = toe systolic pressure ÷ highest brachial pressure; a TBI below 0.70 is diagnostic of peripheral arterial disease. Absolute toe pressures guide healing: > 55 mmHg — wound healing likely; 30–55 mmHg — impaired healing; < 30 mmHg — chronic limb-threatening ischaemia with healing unlikely without revascularisation (a threshold of < 50 mmHg is used in diabetes). Toe pressures are therefore indicated whenever the ABI exceeds 1.40, in all patients with diabetes or renal failure, and to assess healing potential of a foot ulcer or the level of a planned amputation. Complementary tests include Doppler waveform analysis (triphasic → biphasic → monophasic → damped), segmental pressures with pulse volume recording, transcutaneous oxygen pressure (TcPO ), duplex ultrasound, CT or MR angiography.

The 2024 ACC/AHA Guideline for the Management of Lower Extremity Peripheral Artery Disease restructures PAD around four clinical subsets — asymptomatic, chronic symptomatic, chronic limb-threatening ischaemia and acute limb ischaemia. Key diagnostic recommendations: the resting ABI remains the first-line test, but toe brachial index or other physiological testing is explicitly recommended when the ABI is greater than 1.40 or is normal despite a high clinical suspicion, and a wound, ischaemia and foot infection (WIfI) staging should be applied to every threatened limb. For treatment, it emphasises structured supervised exercise therapy as first-line for claudication, high-intensity statin plus antiplatelet therapy for all patients, and — importantly — low-dose rivaroxaban 2.5 mg twice daily plus aspirin (the COMPASS/VOYAGER-PAD regimen) to reduce major adverse limb and cardiovascular events, weighed against bleeding risk in the older patient.