ABI and TBI Testing — What Each Exam Measures

Ankle-brachial index and toe-brachial index testing are the two studies most often confused with each other by the administrative side of a practice, largely because they sound like variants of the same thing. They are related, they are frequently ordered together, and they are not interchangeable. Understanding what each one measures makes scheduling, room setup and patient preparation considerably easier.

This article explains what each exam measures and what performing them on site involves. It does not address how results are interpreted — that is the interpreting physician's work, and nothing here should be read as clinical guidance.

What an ABI measures

The ankle-brachial index is a comparison of blood pressure measured at the ankle against blood pressure measured at the arm. The technologist places cuffs, uses a Doppler probe to identify the signal at the relevant vessels, records systolic pressures, and calculates the ratio between the ankle and brachial measurements for each side.

Operationally, it is a pressure test rather than an imaging study. No pictures of the arteries are produced by the index itself. That is why practices frequently pair it with lower extremity arterial duplex ultrasound, which images the arterial segments directly. The two answer different questions and the combination is common rather than redundant.

What a TBI measures

The toe-brachial index applies the same principle further down the limb: pressure measured at the toe compared against the arm, using a small digit cuff and a sensor appropriate to that measurement. It exists because there are patients in whom ankle-level pressure measurement is not informative, and in those patients a toe-level measurement provides information the ankle measurement cannot.

For a practice, the useful thing to know is that a TBI takes longer per limb than an ABI, needs additional small-cuff equipment, and is more sensitive to a cold room — digit measurement in a patient with cold feet is difficult. Practices that add TBI to their standing protocol should plan for warm-up time in the appointment rather than treating it as a two-minute addendum.

Side by side

• Measurement site: ABI at the ankle; TBI at the toe.
• Reference: both compare against brachial systolic pressure at the arm.
• Equipment: ABI uses standard limb cuffs and a Doppler probe; TBI additionally requires digit cuffs and an appropriate sensor.
• Time: TBI adds meaningfully to the appointment, particularly bilaterally.
• Room conditions: TBI is affected by peripheral temperature; a cold room makes the study harder.
• Ordered together: frequently, because they measure at different levels of the same limb.

What the appointment involves

1. 1
   Patient preparation
   Shoes and socks off, lower legs and one arm accessible. Patients rest supine for a period before measurement — a study performed immediately after the patient walked in from the car parking area is not performed under the conditions the protocol assumes.
2. 2
   Cuff placement
   Cuffs are placed at the arm and at the ankle, plus the digit for TBI. Wound dressings, casts and open ulcers change what is possible, which is why the technologist needs to know about them in advance.
3. 3
   Signal acquisition and pressure recording
   The Doppler probe locates the signal and systolic pressures are recorded at each level, on both sides.
4. 4
   Calculation and documentation
   Indices are calculated and recorded on the worksheet with the underlying pressures, so the interpreting physician sees the measurements rather than only the ratio.
5. 5
   Handoff
   The worksheet goes to the physician responsible for the patient's care for interpretation.

Which practices order these studies

Podiatry offices are the heaviest users, particularly practices with active wound care and diabetic foot caseloads — the operational side of adding these studies to a podiatry office is covered in bringing vascular imaging into a podiatry practice. Primary care offices order index testing as part of general circulation assessment. Skilled nursing facilities and rehabilitation centres order it for residents under wound care, where the alternative is sending someone out of the building for a study that takes less time than the transport does; that workflow is described in how mobile vascular ultrasound works in a nursing home or SNF. Vascular and interventional practices order it alongside duplex imaging.

Screening context

ABI also appears as a component of our preventative screening work, where it is performed alongside carotid, cardiac and aortic screening in community and facility settings. That is a distinct offering from diagnostic index testing ordered by a physician for a specific patient question — details are on the preventative vascular and cardiac screening page.

Performing index testing on site

Because index testing needs no imaging suite, it is among the simplest studies to bring into a building. Any exam room where a patient can lie flat with both feet accessible will do. That simplicity is why practices frequently start here: index testing establishes the imaging day in the schedule, and duplex studies get added once the block is filling reliably.

We perform index testing throughout Chicagoland — Orland Park, Tinley Park, Oak Lawn, Palos Heights, Naperville, Oak Brook, Joliet, Aurora, Elgin and Evanston — and across Northwest Indiana including Hammond, Munster, Schererville, Merrillville, Crown Point, Valparaiso and Michigan City. The exam page for ankle-brachial and toe-brachial index testing covers the study specifically, and the complete mobile ultrasound service list shows what else can be performed in the same visit.

Who performs the studies

Index testing is performed by credentialed, RVT-certified technologists, and our vascular technologists trained through the Rush University vascular ultrasound program.

Practical takeaway

Treat ABI and TBI as two studies, not one with an optional extra. Build the appointment slot around the rest period and, if TBI is included, around the temperature of the room and the extra per-limb time. Practices that do this find index testing the easiest imaging to run in-house; practices that schedule it as a five-minute add-on find it consistently disruptive.

Facilities weighing whether other studies belong in the same visit may find our overview of what a carotid duplex study involves useful, since the two are often ordered for the same patient population.

This article is general information for healthcare professionals about imaging logistics and does not constitute clinical guidance. Studies are interpreted by the physician responsible for the patient's care.