Toe Brachial Index (TBI)

What is Toe Brachial Index?

The Toe Brachial Index (TBI) is defined as the ratio between the systolic blood pressure in the right or left toe and the higher of the systolic pressure in the right or left arms.

Highly sensitive PPG toe clips

TBI is a common vascular physiologic assessment test taken to determine the existence and severity of peripheral arterial disease (PAD) in the lower extremities.

A peripheral arterial disease in the lower extremities happens when the blood vessels in the legs or towards the legs are narrowed or blocked. This most commonly occurs due to atherosclerosis, which is a thickening or hardening of the arteries as a result of fatty plaque building up in the arteries. PAD can also happen in the arms, but it is most common in the legs.

When is a TBI Test Recommended?

Frequently, Toe-Brachial Index (TBI) is a complementary measurement to the ABI (Ankle-Brachial Index) test. It is performed when the ABI cannot be measured, if it is inconclusive, or if it is abnormally high, for example, in diabetic patients. Also, it is recommended to perform a Toe Brachial Index test in case of leg pain during walking and when there are known risk factors for Peripheral Artery Disease. 

Early detection of PAD is critical. If left untreated, it may lead to a Critical Limb Ischemia (CLI). In this condition, severe blockages in the lower extremity arteries prevent enough oxygenated blood from reaching the legs. This poor circulation may result in leg pain, claudication, and in severe cases even Gangrene (broken and dead tissue) and a need for amputation.

TBI measurements are fast, simple, non-invasive, and painless and are widely popular to assist in the early detection of Peripheral Artery Disease.

What is ABI and TBI test?

ABI tests have an important role in diagnosing peripheral arterial disease. 

The TBI measurements in the right/left toes are often taken if the ABI measurements are high (for example, above 1.3) or if the ankle’s arteries are determined as incompressible when measuring the ABI.

While the Ankle Brachial Index measurement can become unreliable when arterial stiffness increases, the Toe Brachial Index is less susceptible to vascular stiffness. This distinction is because an important characteristic of the digit or toe arteries is that they usually do not calcify and therefore do not have increased vascular stiffness. Thus, the TBI is a valuable diagnostic tool when the blood vessels are stiff or incompressible, as may be the case, for example, when diagnosing patients with Diabetes Mellitus (DM).

How to Measure Toe Brachial Index

Measuring the TBI as part of Peripheral Arterial Disease (PAD) evaluation is fast and simple.

In preparation for the test, wrap a toe/digit blood pressure cuff around the measured toe. Use blood pressure cuffs in a size that is suitable for the toe. Then, place a sensor, normally a Photoplethysmograph (PPG) sensor based on infrared light, on the toe distally to the small blood pressure cuff. In some special cases, you can use a Doppler measurement instead of the PPG measurement to perform the test.

Inflate the toe pressure cuff above normal systolic blood pressure until the distal PPG signal flattens, indicating total arterial occlusion.

Toe Measurement using a Digit Blood Pressure Cuff and a PPG Sensor

Toe Measurement Using Digit Cuff and PPG Sensor

The blood pressure cuff is then deflated in a controlled manner, and the pressure at which the phasic PPG waveform reappears is determined as the systolic pressure. 

Due to the noisy PPG signal environment, high-end ABI machines also present the “average” PPG curve (or DC PPG) curve. The average curve tends to rise at the correct systolic return. Using this method, technicians can easily determine the systolic blood pressure location.

Toe Measurement with DC PPG Curve

Toe Brachial Index with DC PPG

Finally, repeat the process described above for the 2 Brachial segments and for 2 toes. 

How is the TBI Calculated?

Usually, an Ankle Brachial Index (ABI) test is also carried out, which means the Brachial segments have already been measured. The highest of the two Brachial systolic pressures is used as a reference. 

The Toe-Brachial Index (TBI) is calculated as follows:

Toe Brachial Index (TBI) Formula
PPG Toe Clips

PPG Sensors

Secondary Method of ABI Assessment

Cuffs for Falcon Products

Inflatable Cuffs

High quality available in a variety of sizes

Using the Falcon for TBI Measurements

The Falcon as a TBI Machine is designed to allow completing the Toe Brachial Index diagnosis in a fast and straightforward manner. All that is required is as follows:

  • Wrap the toe/digit cuff around the toe of interest.
  • Place a PPG sensor distal to the cuff on the tip of the toe.  

The Falcon provides a variety of PPG sensors that can be used to meet particular needs: 

  • The fingerclip PPG sensor can be used for the digits or long toes, 
  • The toe clip PPG sensors can be used for rapid sensor placement on the toes, and
  • In short toes with limited space distal to the pressure cuff, the disk PPG sensor with a special adhesive sticker can be placed on the tip of the toe.  

The Falcon allows performing pressure measurements in up to 5 separate toes simultaneously. For example, this will enable us to complete the right and left toe pressure measurement bilaterally to save examination time. 

In the special cases where Doppler is selected for the TBI test, the Falcon allows selection from a range of Doppler probe frequencies, including the 10MHz probe for superficial penetration. 

All PPG sensors, probes, and pressure tubes are color-coded to allow optimal user interface and correlation with the dedicated software. The user can easily change the target inflation pressure, as well as the deflate rate and various other parameters for optimal measurements. 

Once the measurement is completed, the TBI index appears in the examination report. If multiple toes are measured in each foot, then the separate pressure index of each measured toe is clearly displayed on the screen and included in the report.  

The Falcon supports many other options and features which are designed to simplify the use of the Falcon TBI physiologic diagnostic system in a fast and efficient way. One such feature is displaying the “average” PPG waveform, which is a function of total blood flow, rather than showing only the phasic waveform. This additional curve provides a valuable supplementary criterion when determining the systolic pressure, particularly in a noisy environment.  

Once the distal PPG (or Doppler) waveform reappears during the pressure cuff deflation phase, the measurement can be stopped, and the Falcon places a cursor to mark a potential systolic pressure. The user may move the cursor to a different location, and the systolic pressure is changed accordingly. R/L TBI values are calculated automatically by the Falcon system and presented clearly on the screen and in the report layout.

Expected Results for TBI

Although the pathologic TBI criteria are still somewhat ambiguous, a common Toe Brachial Index normal values range is shown in the table below.

RangeCommon Diagnosis
TBI ≥ 0.7Normal
TBI < 0.7Abnormal

Specifically, TBI < 0.7 is associated with claudication, and a TBI < 0.2 is associated with pain at rest.

The absolute systolic toe pressure is also an important diagnostic parameter for Peripheral Arterial Disease. A normal toe pressure is about 30 mmHg less than the ankle pressure. A systolic toe pressure of less than 30 to 50 mmHg suggests chronic limb ischemia, according to the recommendations of the TASC group. 

A TBI value less than 0.5 suggests proximal arterial disease of moderate severity, while an index less than 0.2 and toe pressures less than 30 mm Hg are consistent with critical ischemia and poor potential for healing.

The TBI normal values are different from the ABI normal values because the toe blood pressures are expected to be less than ankle/brachial blood pressures.

Selected Literature

Overview of Peripheral Arterial Disease of the Lower Extremity, Ali F. AbuRahma and John E. Campbell, Noninvasive Vascular Diagnosis, A.F. AbuRahma (ed.), Springer International Publishing AG 2017, Ch 21, pp 291-318

2016 AHA/ACC Guideline on the Management of Patients With Lower Extremity Peripheral Artery Disease; A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines; Journal of the American College of Cardiology, Vol. 69, No. 11, 2017

Peripheral vascular disease assessment in the lower limb: a review of current and emerging non‑invasive diagnostic methods, Shabani Varaki et al, BioMed Eng OnLine (2018) 17:61

2017 ESC Guidelines on the Diagnosis and Treatment of Peripheral Arterial Diseases, in collaboration with the European Society for Vascular Surgery (ESVS), Victor Aboyans et al., European Heart Journal (2017) 00, 1–60

The Ability of the Toe-Brachial Index to Predict the Outcome of Treadmill Exercise Testing in Patients with a Normal Resting Ankle-Brachial Index, Christian Høyer et al, Annals of Vascular Surgery, Volume 64, April 2020, Pages 263-269

Association of cardiovascular autonomic dysfunction with peripheral arterial stiffness in patients with type 1 diabetes, Lía Nattero-Chávez et al, The Journal of Clinical Endocrinology & Metabolism

The toe-brachial index in the diagnosis of peripheralarterial disease, Christian Høyer et al, Journal of Vascular Surgery, Volume 58, Issue 1, P231-238, July 01, 2013

Inter Society Consensus Report for the Management of PAD, TASC II report (Trans-Atlantic Inter-Society Consensus), Ed. Lars Norgren and William R Hiatt, The Society for Vascular Surgery, doi:10.1016.

Peripheral vascular disease assessment in the lower limb: a review of current and emerging non‑invasive diagnostic methods; Shabani Varaki et al, BioMed Eng OnLine (2018) 17:61

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