Ankle-Brachial Index — Doppler measurement of limb systolic pressures to diagnose and grade peripheral artery disease
The ankle-brachial index begins in the arm. A properly sized blood pressure cuff is placed around each upper arm, and a handheld continuous-wave Doppler probe (5–10 MHz) is used — instead of a stethoscope — to detect the return of arterial flow signal as the cuff deflates. This substitution of Doppler for auscultation is what allows the same technique to later work at the ankle, where Korotkoff sounds are usually inaudible.
Brachial systolic pressure is measured in both the right and left arms, and the higher of the two values is used as the denominator for every ABI calculation on that patient — regardless of which leg is being assessed. This convention exists because a clinically silent subclavian or innominate artery stenosis can artificially lower pressure in one arm, which would falsely deflate the ABI if that arm were used as the reference.
A between-arm systolic difference greater than 10–15 mmHg is itself a recognized marker of increased cardiovascular risk and can indicate proximal upper-extremity arterial disease, so both arms are measured on every occasion, not assumed to be equal from a prior visit.
The technique replaces the stethoscope with a Doppler probe angled roughly 45–60° to the skin over the palpated brachial artery pulse, usually just medial to the biceps tendon at the antecubital fossa. Ultrasound gel couples the probe to the skin. The cuff is inflated 20–30 mmHg above the point where the audible arterial signal disappears, then slowly deflated (2–3 mmHg per second).
The systolic pressure is recorded at the exact cuff pressure where the Doppler "whoosh" signal reappears — this is the reappearance of antegrade flow, not a change in sound quality as with Korotkoff sounds. Because Doppler only detects the presence or absence of flow (not the turbulent vibrations of Korotkoff sounds), diastolic pressure cannot reliably be determined this way — only systolic pressure is used for ABI.
Patient positioning matters: measurements are taken supine after at least 10 minutes of rest, with cuffs at heart level, in a warm quiet room. Recent smoking, caffeine, exercise, or cold extremities can transiently alter peripheral pressures and should be avoided before testing.
The same cuff-and-Doppler technique is repeated at the ankle, but now two arteries are interrogated per leg: the dorsalis pedis, on the dorsum of the foot, and the posterior tibial, behind the medial malleolus. The higher systolic pressure of the two becomes the numerator for that leg's ABI — each leg is scored independently, producing a left ABI and a right ABI.
The ankle cuff is wrapped directly above the malleoli. The dorsalis pedis artery is found lateral to the extensor hallucis longus tendon on the dorsum of the foot; the posterior tibial artery runs just posterior and inferior to the medial malleolus. Both are superficial and readily located by palpation before the Doppler probe confirms an audible signal.
As with the brachial measurement, the cuff is inflated until the Doppler signal disappears, then deflated slowly; systolic pressure is the cuff pressure at which flow signal reappears. This is repeated for both the dorsalis pedis and posterior tibial arteries on each leg — four ankle readings total, two per side.
Anatomic variants (a congenitally absent or hypoplastic dorsalis pedis occurs in roughly 2–3% of limbs) and focal atherosclerotic disease can silence one pedal vessel while the other remains patent. Taking the higher of the two per leg avoids penalizing a limb for an isolated distal branch occlusion that does not reflect overall inflow, while still capturing genuine multivessel disease when both signals are low. The lower of the two values is sometimes recorded separately in research settings because it can carry additional prognostic weight for tissue-loss risk, but the American Heart Association standard for the diagnostic ABI uses the higher pressure per leg.
With both inputs in hand, the ankle-brachial index reduces to a single division: the higher systolic pressure recorded at that leg's ankle, divided by the higher systolic pressure recorded across both arms. The result is a unitless ratio — normally slightly above 1.0, because in a healthy supine person the legs are perfused at least as well as the arms.
For the right leg: ABI(right) = higher(right DP, right PT) ÷ higher(left brachial, right brachial). For the left leg: ABI(left) = higher(left DP, left PT) ÷ higher(left brachial, right brachial). Both legs share the same denominator — the single highest brachial reading obtained anywhere in the exam.
Example: right arm 132 mmHg, left arm 140 mmHg → denominator 140 mmHg. Right dorsalis pedis 100 mmHg, right posterior tibial 118 mmHg → numerator 118 mmHg. ABI(right) = 118 / 140 = 0.84, consistent with mild-to-moderate peripheral artery disease in that limb even though the contralateral leg could be entirely normal.
Using a ratio normalizes ankle pressure to each individual's own central arterial pressure, so a patient with baseline hypertension and a patient with baseline hypotension are compared on the same relative scale rather than by raw millimeters of mercury. This is what makes a single threshold (roughly 1.0) meaningful across a diverse population, and why ABI — not ankle pressure alone — is the reported diagnostic quantity in essentially every vascular guideline.
The numeric ABI is mapped onto a standardized five-tier interpretation scale adopted by the American Heart Association and endorsed by vascular medicine societies worldwide. Each tier carries distinct implications for symptom likelihood, further testing, and management urgency — and the extremes of the scale both signal disease, just through opposite mechanisms.
An ABI above 1.40 does not indicate healthy vessels — it indicates that the vessel could not be compressed by the cuff, most often because of medial arterial calcification (Mönckeberg sclerosis) common in diabetes and end-stage renal disease. These values are unreliable and require an alternative test.
An ABI between 1.00 and 1.40 is normal. Between 0.91 and 0.99 is borderline/equivocal — often prompting a post-exercise ABI, since PAD can unmask itself only under the metabolic demand of walking. From 0.41 to 0.90, mild-to-moderate PAD is present, generally correlating with intermittent claudication. At or below 0.40, severe disease is present, consistent with rest pain, non-healing ulceration, or frank critical limb ischemia.
ABI interpretation table — ABI >1.40: Non-compressible / calcified, unreliable (order toe-brachial index). ABI 1.00–1.40: Normal. ABI 0.91–0.99: Borderline, equivocal. ABI 0.41–0.90: Mild–moderate PAD, claudication likely. ABI ≤0.40: Severe PAD / critical limb ischemia, urgent vascular referral.
Because medial calcification stiffens the arterial wall without necessarily obstructing the lumen, the cuff cannot fully compress a calcified tibial artery, producing an artifactually high pressure reading and an ABI that overstates perfusion — sometimes masking severe underlying stenosis. This is especially common in long-standing diabetes mellitus and chronic kidney disease/dialysis populations.
When ABI exceeds 1.40, or when a diabetic patient's ABI is unexpectedly normal despite classic symptoms, the toe-brachial index (TBI) is the recommended alternative: digital arteries of the toes are rarely calcified to the same degree, so a small pneumatic cuff on the great toe with photoplethysmography can still yield an interpretable ratio (normal TBI ≥0.70).
The ABI is rarely an isolated data point — it is interpreted alongside claudication symptoms, cardiovascular risk factors, and physical exam findings to decide what happens next. An abnormal ABI is also, independently of leg symptoms, one of the strongest simple predictors of systemic atherosclerotic burden and future cardiovascular events.
Mild-to-moderate PAD (ABI 0.41–0.90) typically correlates with intermittent claudication — reproducible calf, thigh, or buttock pain on walking that resolves with rest — though roughly half of people with an abnormal ABI report no classic symptoms at all and are identified only through screening in high-risk groups (age ≥65, diabetes, smoking history, known coronary or cerebrovascular disease).
Severe disease (ABI ≤0.40) is associated with rest pain, non-healing wounds, and gangrene — the clinical syndrome of chronic limb-threatening ischemia (CLTI) — where the limb itself, not just walking distance, is at risk. A low ABI is also an independent marker of systemic atherosclerosis: patients with PAD carry a markedly elevated risk of myocardial infarction, stroke, and cardiovascular death even if their leg symptoms are mild.
An abnormal or equivocal ABI generally leads to duplex ultrasound of the lower extremity arteries to localize and grade stenoses anatomically, with CT angiography or catheter-based digital subtraction angiography reserved for pre-procedural planning when revascularization is being considered.
Management is graded to severity: borderline/mild disease is managed first with risk-factor modification (smoking cessation, statin therapy, antiplatelet therapy, glycemic and blood pressure control) plus a supervised exercise therapy program, which reliably improves walking distance. Moderate-to-severe claudication unresponsive to exercise and risk-factor control, and any critical limb ischemia, prompts referral for endovascular (angioplasty/stenting) or surgical (bypass) revascularization to prevent tissue loss and preserve the limb.
Because PAD is a marker of systemic vascular disease, guideline-directed medical therapy — statin, antiplatelet agent, blood pressure control, and smoking cessation — is indicated for every patient with an abnormal ABI regardless of whether revascularization is pursued, to reduce the elevated risk of MI, stroke, and cardiovascular death.