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ABI

General

Ankle-Brachial Index

A ratio comparing blood pressure in the ankle to blood pressure in the arm, used as a simple non-invasive screening tool for peripheral artery disease.

Definition

ABI (Ankle-Brachial Index) is a simple, non-invasive test that compares the blood pressure measured at the ankle to the blood pressure measured at the arm (brachial artery). It is the primary screening tool for peripheral artery disease (PAD) — a condition in which arteries supplying the legs become narrowed or blocked by plaque buildup, reducing blood flow.

ABI is used to:

  • Screen for and diagnose peripheral artery disease (PAD)
  • Assess risk of leg pain, non-healing wounds, and amputation risk in advanced cases
  • Flag increased cardiovascular risk, since PAD often coexists with coronary and carotid artery disease
  • Monitor disease progression or treatment response over time

Because leg arteries affected by PAD have to work harder to maintain blood flow, systolic pressure at the ankle tends to be lower relative to the arm when PAD is present — which is exactly what the ABI ratio detects.

Formula

ABI = Ankle Systolic Pressure ÷ Brachial (Arm) Systolic Pressure

The ABI is calculated separately for each leg, using the higher systolic pressure reading between the two arms as the denominator for both calculations.

Worked Example

A patient's blood pressure readings are:

  • Right arm systolic pressure: 130 mmHg
  • Left arm systolic pressure: 128 mmHg (use the higher value: 130 mmHg)
  • Right ankle systolic pressure: 100 mmHg

Right ABI = 100 ÷ 130 = 0.77

Interpretation: A right ABI of 0.77 falls below the 0.90 threshold, indicating peripheral artery disease in the right leg and warranting further clinical evaluation. Use the ABI calculator to compute and interpret ABI for both legs from your blood pressure readings.

Key Things to Know

  • Interpretation ranges are standardized: 1.0-1.4 is normal, 0.91-0.99 is borderline, 0.90 or below indicates PAD (with lower values indicating more severe disease), and above 1.4 suggests non-compressible, calcified arteries requiring alternative testing.
  • ABI is calculated per leg: Because PAD can affect one leg more than the other, ABI should be calculated and interpreted separately for the left and right leg, not averaged together.
  • Diabetes can distort results: Arterial calcification common in long-standing diabetes can produce falsely elevated (incompressible) ABI readings, masking underlying PAD — additional tests are needed in these patients.
  • PAD signals broader cardiovascular risk: An abnormal ABI often prompts evaluation for coronary artery disease and stroke risk as well, since atherosclerosis is typically a systemic process affecting multiple arterial beds, not just the legs.
  • ABI and MAP use the same tools differently: Both rely on a blood pressure cuff, but ABI compares two body sites to detect localized blockages, while MAP summarizes overall central arterial pressure driving whole-body perfusion.

Related Calculators

ABI Calculator (Ankle-Brachial Index) →Calculate the ankle-brachial index (ABI) for each leg from brachial and ankle systolic pressures, with peripheral artery disease severity bands shown.
MAP Calculator (Mean Arterial Pressure) →Calculate mean arterial pressure (MAP) from systolic and diastolic blood pressure to check organ perfusion adequacy, with normal range context.

Related Terms

MAPMean Arterial PressureBMIBody Mass Index

Frequently Asked Questions

A normal Ankle-Brachial Index falls between 1.0 and 1.4, meaning ankle blood pressure is equal to or slightly higher than arm blood pressure. Values between 0.91 and 0.99 are considered borderline, values at or below 0.90 indicate peripheral artery disease (PAD), and values above 1.4 suggest the arteries may be too stiff to compress normally (common in diabetes), requiring alternative testing. Use the [ABI calculator](/abi-calculator/) to compute and interpret your score.
ABI is measured using a blood pressure cuff and a handheld Doppler ultrasound device to detect blood flow. Systolic blood pressure is measured at both ankles and both arms, and the ABI is calculated for each leg using the higher of the two arm readings as the denominator. The test is non-invasive, takes about 10-15 minutes, and is typically performed in a clinical setting.
A low ABI (0.90 or below) indicates reduced blood flow to the legs, most commonly caused by peripheral artery disease (PAD) — a buildup of plaque narrowing the arteries. PAD increases risk not just for leg symptoms like claudication (pain while walking) but also signals a higher likelihood of blockages elsewhere, including in the coronary and carotid arteries, since atherosclerosis is typically a systemic condition.
Yes. In people with diabetes or advanced kidney disease, arteries can become calcified and stiff, making them resist compression by the blood pressure cuff and producing artificially high or 'incompressible' ABI readings (often above 1.4). In these cases, doctors use alternative tests like toe-brachial index or arterial duplex ultrasound to assess blood flow more accurately.
Both ABI and [Mean Arterial Pressure (MAP)](/glossary/map/) are calculated from blood pressure cuff readings, but they measure different things: MAP summarizes the average pressure driving blood through the whole circulatory system, while ABI compares systolic pressure between two specific sites (ankle and arm) to detect localized blockages in the leg arteries. A patient can have a normal MAP while still having an abnormal ABI indicating PAD.