📄 Clinical Documentation

ABI Calculation Protocol

Ankle Brachial Index & Toe Brachial Index Calculation Guide

Standardized academic protocol based on the 2024 ACC/AHA/Multisociety Guidelines for Lower Extremity PAD.

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✅ ABI Calculation Formula

The Ankle-Brachial Index (ABI) is an essential diagnostic tool for Lower Extremity Peripheral Artery Disease. According to the 2024 ACC/AHA/Multisociety Guidelines, the ABI is calculated separately for each leg. The standard academic formula divides the highest systolic pressure at the ankle by the highest systolic pressure of the brachial arteries.

ABI = (Highest Ankle Pressure) / (Highest Brachial Pressure)

Numerator (Top): The highest systolic pressure measured at the ankle of the leg being assessed. Clinicians must measure both the Dorsalis Pedis (DP) and Posterior Tibial (PT) arteries and utilize the higher of the two values to avoid underestimating distal perfusion.
Denominator (Bottom): The highest systolic brachial pressure measured in either the right or the left arm. This identical denominator is used to calculate the ABI for both the left and the right leg, serving as the systemic pressure reference.

✔ ABI Clinical Interpretation

ABI Value	Interpretation	Clinical Significance
1.00 – 1.40	Normal	Normal arterial flow. No further testing needed unless symptomatic.
0.91 – 0.99	Borderline / Acceptable	May indicate early PAD; consider exercise ABI if symptomatic.
0.71 – 0.90	Mild PAD	Typically asymptomatic or associated with intermittent claudication.
0.41 – 0.70	Moderate PAD	More frequent claudication, reduced walking distance.
≤ 0.40	Severe PAD	Critical limb-threatening ischemia (CLTI); ischemic rest pain, tissue loss.
> 1.40	Non-compressible	Arterial calcification (diabetes, CKD). Unreliable, use TBI.

Source: 2024 ACC/AHA Guidelines for Management of Lower Extremity PAD.

✅ TBI Calculation Formula

The Toe-Brachial Index (TBI) is the preferred diagnostic modality when ABI values are noncompressible (>1.40) due to medial arterial calcification, commonly seen in patients with diabetes mellitus or end-stage renal disease.

TBI = (Toe Systolic Pressure) / (Highest Brachial Pressure)

Numerator (Top): The systolic pressure measured at the great toe (hallux) using a specialized digital cuff and photoplethysmography (PPG) or a continuous wave Doppler. Digital arteries are typically spared from medial calcification.
Denominator (Bottom): The highest systolic brachial pressure measured in either the right or the left arm (identical to the ABI denominator).

✔ TBI Clinical Interpretation

TBI Value	Interpretation	Clinical Significance
≥ 0.70	Normal	Normal arterial flow to the digits.
≤ 0.70	PAD Diagnosis	Diagnostic for Peripheral Artery Disease (PAD).
< 0.30	Severe PAD / Ischemia	Critical limb ischemia; significantly increased risk of non-healing ulcers.

Standard clinical guidelines for TBI interpretation. Often used in cases of medial calcinosis where ABI is unreliable.

✔ Academic Sources & Clinical Practice

• Fast Science - Open Research: Independent clinical practice observations regarding severe obstruction flow loss due to calf pressure against examination bed, and the mechanical obstruction of Anterior Tibial/Dorsalis Pedis arteries by the rigid retinaculum during extreme flexion/extension.
• 2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS Guideline for the Management of Lower Extremity Peripheral Artery Disease.
• American Heart Association Scientific Statement on Measurement and Interpretation of the Ankle-Brachial Index (Circulation).
• Stanford Medicine 25: Ankle Brachial Index Protocol.

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