What is the Ankle-Brachial Index (ABI)?

The Simple Definition of ABI

The Ankle-Brachial Index (ABI) is a ratio — specifically, it's the systolic blood pressure measured at your ankle divided by the systolic blood pressure measured at your upper arm (brachial artery). The word "brachial" simply refers to the arm. The word "index" means it's a comparative ratio, not an absolute value.

Patient learning about the Ankle-Brachial Index (ABI) test and vascular screening guide.

Here's the insight behind the test: in a healthy circulatory system, blood pressure in your ankle should be roughly equal to — or even slightly higher than — the pressure in your arm. Your heart pumps blood downward through the aorta, through the leg arteries, and down to the feet. If those arteries are healthy and wide open, pressure is maintained all the way down. But if there's a narrowing or blockage anywhere along that path — in the iliac artery of your pelvis, the femoral artery of your thigh, or the popliteal artery behind your knee — pressure drops below the blockage, like water pressure dropping after a kinked garden hose.

That drop in pressure is exactly what the ABI captures. And it makes the ABI one of the most elegant screening tests in all of medicine: cheap, fast, painless, and highly informative.

Why the ABI Matters — Beyond Just Leg Health

Here's something that surprises many people: a low ABI doesn't just tell you about your legs. It tells you about your entire cardiovascular system. Because the same process — atherosclerosis — that blocks leg arteries also tends to block coronary arteries (causing heart attacks) and carotid arteries (causing strokes).

Multiple large studies have confirmed that people with a low ABI (below 0.9) are 2 to 4 times more likely to experience a major cardiovascular event — heart attack, stroke, or cardiovascular death — over the next 10 years compared to people with a normal ABI. This makes the ABI a powerful cardiovascular risk marker, not just a leg health test.

According to data published in the Journal of the American College of Cardiology, when ABI is added to the standard Framingham Risk Score calculation, it significantly improves the ability to predict who will have a heart attack in the next decade. The UK's NICE guidelines and the American Heart Association both recommend considering ABI as part of cardiovascular risk assessment in borderline-risk patients.

In Canada, the Canadian Cardiovascular Society guidelines similarly endorse the ABI as a useful tool for identifying patients who may benefit from more aggressive lipid-lowering or antiplatelet therapy — patients who would otherwise appear low-risk by traditional metrics alone.

The ABI Formula — Step by Step

The math is refreshingly simple:

In a clinical setting, the measurement is done on both sides:

• Blood pressure is measured in both arms (left and right brachial arteries)
• Blood pressure is measured at both ankles using a Doppler probe — typically at the posterior tibial artery and the dorsalis pedis artery
• The highest arm pressure is used as the denominator
• For each leg, the highest ankle pressure on that side is divided by that highest arm pressure
• This gives you two ABI values — one for each leg. The lower of the two is typically the clinically significant number

Our free ABI calculator handles this in seconds once you have your pressure readings.

A Brief History of the ABI Test

The ABI test has surprisingly deep roots. The concept of comparing limb blood pressures dates back to work by physiologists in the early 20th century, but the modern ABI test as we know it was developed in the 1950s and 1960s — coinciding with the advent of the hand-held Doppler ultrasound probe.

Scottish vascular surgeon Andrew Nicolaides and American researchers including Donald Strandness were pivotal in establishing the clinical utility and reference ranges for ABI in the 1960s and 1970s. Their work showed that an ABI below 0.9 was a clinically meaningful threshold for identifying PAD, and this cut-point has remained remarkably stable in the guidelines ever since.

The test gained mainstream clinical prominence after the publication of the Edinburgh Artery Study in the late 1980s — one of the largest population-based studies ever conducted on vascular disease — which used ABI as its primary measure and revealed just how common asymptomatic PAD was in the general population.

What the ABI Cannot Tell You

The ABI is powerful, but it has real limitations that are important to understand:

• Non-compressible arteries: In patients with diabetes, advanced kidney disease, or the very elderly, the arteries can become hardened and calcified to the point where a blood pressure cuff cannot compress them. This produces falsely high ABI readings (above 1.40), making the test unreliable. In these patients, a Toe-Brachial Index (TBI) is preferred.
• Location of disease: The ABI confirms that a blockage exists and how severe it is — but it doesn't tell you exactly where the blockage is. For that, you'd need an imaging test like a duplex ultrasound, CT angiogram, or MR angiogram.
• Intermittent disease: Some patients have normal resting ABI but develop symptoms and pressure drops only during exercise. For these patients, an exercise ABI test (where measurements are taken before and after a treadmill walk) is more revealing.
• Not a heart-specific test: While a low ABI indicates systemic cardiovascular risk, it doesn't directly measure coronary artery disease. Additional cardiac testing is needed for a full cardiovascular workup.

Who Uses the ABI in Clinical Practice?

The ABI isn't just a cardiologist's tool. It is used across multiple medical specialties:

• Primary care physicians and internists use ABI for routine vascular screening in high-risk patients
• Vascular surgeons and interventional radiologists use it to quantify disease severity before and after procedures
• Diabetologists and endocrinologists use it (or TBI) to screen for vascular complications in diabetic patients
• Wound care specialists use ABI to assess whether a patient has adequate blood flow to heal a wound before applying compression therapy
• Cardiologists use it as a cardiovascular risk marker
• Physiotherapists and exercise rehabilitation specialists use it to baseline a patient's vascular status before starting supervised exercise programs

Compression therapy — used to treat venous leg ulcers — is actually contraindicated in patients with significant PAD (generally ABI below 0.8). This is why wound care nurses often measure the ABI before applying compression bandages: applying strong compression to a limb with compromised arterial inflow can cause serious harm.

Frequently Asked Questions