ABI Medical Abbreviation: Reading the Ankle Brachial Index

What the ABI medical abbreviation measures

Healthy arteries carry blood to the ankles at roughly the same pressure as to the arms, sometimes a little higher. When plaque narrows the arteries of the legs, pressure drops beyond the narrowing. The ankle brachial index captures that drop as a single number.

The word brachial refers to the upper arm, where the brachial artery runs. So the name literally means an index comparing ankle and arm. The test is a common first step when someone has calf cramping on walks that settles after a short stop, slow healing foot wounds, or risk factors such as smoking, diabetes or high blood pressure.

How the test is done

The person lies flat and rests for several minutes. A technician or nurse places blood pressure cuffs on both upper arms and just above both ankles. A small handheld ultrasound probe listens for blood flow in the arteries of the arm and foot as each cuff is slowly released, and the pressure at which flow returns is recorded.

For each leg, the higher of the two ankle pressures is divided by the higher of the two arm pressures. The whole test usually takes about 15 to 30 minutes and involves no needles. Some labs also repeat the ankle readings after a short walk on a treadmill, which can unmask narrowing that is mild at rest.

Reading ABI numbers

Most vascular labs use similar cutoffs, though wording varies. The table gives the usual broad bands. Your own report and your clinician will say how they apply to you.

A very high value is common in people with long standing diabetes or kidney disease, whose artery walls can become calcified. In that case a toe pressure or toe brachial index may be measured instead.

Acquired brain injury: the other ABI

In rehabilitation hospitals, neurology and community brain injury services, ABI often means acquired brain injury. That is any brain injury that happens after birth, whether from trauma, stroke, lack of oxygen, infection or a tumor. A TBI, or traumatic brain injury, is one type of acquired brain injury.

Context tends to give it away. A number such as 0.85 points to the vascular test, while phrases about cognition, therapy goals or community reentry point to brain injury. If a note uses ABI without a number in a mixed setting, ask the author which one is meant.

Where ABI fits in a vascular workup

An abnormal index is often followed by more detailed imaging to find where the narrowing sits. That may be a duplex ultrasound of the leg arteries, a CTA, or an MRA. A common site of narrowing in the thigh is the SFA, the superficial femoral artery.

Older notes may describe the same condition as PVD, peripheral vascular disease. For more test shorthand like this, browse the lab test and imaging abbreviations.

Questions people often ask after an ABI

• Which leg had the lower number, and how low was it?
• Was the result reliable, or were my arteries too stiff to compress?
• Do I need more imaging to see exactly where the narrowing is?
• How often should the test be repeated?

It helps to wear loose clothing so the cuffs can go on easily and to avoid smoking or caffeine shortly before the test, unless told otherwise.