Toe-Brachial Index (TBI) in Non-Compressible Arteries (Diabetes, CKD): Methodology and Thresholds
Non-Compressibility Phenomenon and TBI Rationale
In patients with diabetes, chronic kidney disease (CKD), and the elderly, medial calcinosis — calcification of the medial layer of the tibial arteries — develops. The wall loses compressibility, the cuff does not occlude the vessel, and the ABI appears falsely normal or even high (>1.40) even with confirmed severe lesions. Since toe arteries are almost unaffected by medial calcinosis, the Toe-Brachial Index (TBI) retains diagnostic value where the ABI is uninformative.
Indications for TBI
TBI is performed when the ABI is >1.40 (a sign of non-compressibility), as well as generally in patients with diabetes where a normal or high ABI does not rule out peripheral artery disease (PAD).
Methodology and Interpretation
TBI is the ratio of the pressure on the first toe to the highest brachial pressure. A threshold value of <0.70 is considered pathological and allows for the diagnosis of PAD in situations where the ABI is uninformative.
| Indicator | Value | Interpretation |
|---|---|---|
| ABI | <0.90 | Peripheral Artery Disease |
| ABI | >1.40 | Non-Compressibility (TBI indicated) |
| TBI | <0.70 | Pathology (PAD) |
Diagnostic Value in Diabetics
In patients with diabetes, TBI is significantly more sensitive than ABI: compared to CT angiography with a formally “normal” ABI, the sensitivity of TBI is about 80% versus about 35% for ABI.
Place in Ischemia Assessment System
ABI and TBI are fundamental to the quantitative assessment of ischemia. The thresholds of ABI/TBI as a measure of the ischemic component are the basis for all classifications of ischemia severity: both the classic clinical stages (Fontaine, Pokrovsky) and the modern WIfI system.
Frequently asked questions
At what ABI is TBI indicated?
When the ABI is >1.40 (a sign of non-compressibility), as well as in patients with diabetes where a normal or high ABI does not rule out PAD.
What TBI threshold is considered pathological?
A TBI <0.70 is considered pathological and allows for the diagnosis of PAD where the ABI is uninformative.
How is TBI calculated?
TBI is the ratio of the pressure on the first toe to the highest brachial pressure.
Why is TBI more informative than ABI in diabetics?
Toe arteries are almost unaffected by medial calcinosis. The sensitivity of TBI with a “normal” ABI is ~80% versus ~35% for ABI (compared to CT angiography).
In which patients is the ABI falsely normal or high?
In patients with diabetes, CKD, and the elderly, due to medial calcinosis of the tibial arteries, the wall loses compressibility, and the ABI appears falsely normal or high (>1.40).