How to Calculate Ankle Brachial Index

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Ankle-Brachial Index (ABI) Calculator

Arm Systolic Pressure (mmHg)

Right Ankle Systolic (mmHg)

Left Ankle Systolic (mmHg)

Results

Right ABI

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Left ABI

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function calculateABI() { var rArm = parseFloat(document.getElementById('rightArm').value); var lArm = parseFloat(document.getElementById('leftArm').value); var rDP = parseFloat(document.getElementById('rightDP').value); var rPT = parseFloat(document.getElementById('rightPT').value); var lDP = parseFloat(document.getElementById('leftDP').value); var lPT = parseFloat(document.getElementById('leftPT').value); if (isNaN(rArm) || isNaN(lArm) || isNaN(rDP) || isNaN(rPT) || isNaN(lDP) || isNaN(lPT)) { alert("Please enter valid numeric values for all fields."); return; } // Standard clinical ABI calculation: // Higher of the two arm pressures is used as the denominator for BOTH legs var higherArm = Math.max(rArm, lArm); // For each leg, use the higher of the DP or PT artery pressures var higherRightAnkle = Math.max(rDP, rPT); var higherLeftAnkle = Math.max(lDP, lPT); var rightABI = higherRightAnkle / higherArm; var leftABI = higherLeftAnkle / higherArm; document.getElementById('rightValue').innerText = rightABI.toFixed(2); document.getElementById('leftValue').innerText = leftABI.toFixed(2); var resultDiv = document.getElementById('abiResult'); var interpretationDiv = document.getElementById('abiInterpretation'); resultDiv.style.display = "block"; var worstABI = Math.min(rightABI, leftABI); var status = ""; var color = ""; if (worstABI > 1.4) { status = "Non-compressible / Calcified Vessels (Possible high risk)"; color = "#9b59b6"; } else if (worstABI >= 1.0) { status = "Normal: No evidence of Peripheral Artery Disease"; color = "#27ae60"; } else if (worstABI >= 0.9) { status = "Borderline / Low Normal"; color = "#f1c40f"; } else if (worstABI >= 0.7) { status = "Mild Peripheral Artery Disease"; color = "#e67e22"; } else if (worstABI >= 0.4) { status = "Moderate Peripheral Artery Disease"; color = "#d35400"; } else { status = "Severe Peripheral Artery Disease"; color = "#c0392b"; } interpretationDiv.innerText = status; interpretationDiv.style.backgroundColor = color; interpretationDiv.style.color = "white"; }

How to Calculate Ankle-Brachial Index: A Clinical Guide

The Ankle-Brachial Index (ABI) is a simple, non-invasive test used to diagnose Peripheral Artery Disease (PAD). By comparing the blood pressure measured at your ankle with the blood pressure measured at your arm, healthcare providers can determine how well your blood is flowing to your limbs.

The ABI Formula

To calculate the Ankle-Brachial Index, the clinician measures the systolic blood pressure in both arms and at both ankles (using the dorsalis pedis and posterior tibial arteries). The formula is applied separately for each leg:

ABI = (Highest Systolic Pressure in Ankle) / (Highest Systolic Pressure in Either Arm)

Step-by-Step Calculation Example

Suppose a patient has the following systolic blood pressure readings:

  • Right Arm: 130 mmHg
  • Left Arm: 135 mmHg (The "Highest Arm" value is 135)
  • Right Ankle: DP is 140, PT is 138 (Highest Ankle is 140)
  • Left Ankle: DP is 110, PT is 105 (Highest Ankle is 110)

Calculating Right ABI: 140 ÷ 135 = 1.04 (Normal)

Calculating Left ABI: 110 ÷ 135 = 0.81 (Mild PAD)

Understanding the Results

ABI Range Interpretation
1.0 – 1.4 Normal
0.91 – 0.99 Borderline
0.41 – 0.90 Mild to Moderate PAD
0.00 – 0.40 Severe PAD
> 1.40 Vessel Wall Stiffness (Calcification)

Who Should Get an ABI Test?

The ABI test is often recommended for individuals who are at risk of Peripheral Artery Disease. Key risk factors include:

  • Being over the age of 50 with a history of smoking or diabetes.
  • Individuals over 70, regardless of risk factors.
  • People with high blood pressure or high cholesterol.
  • Experiencing leg pain while walking (claudication) that stops when resting.

Practical Tips for Accuracy

  1. Resting Period: The patient should rest supine (lying flat on their back) for at least 10–20 minutes before taking measurements.
  2. Cuff Size: Use an appropriately sized blood pressure cuff to avoid "cuff hypertension."
  3. Doppler Probe: In a clinical setting, a handheld Doppler probe is used to listen for the pulse to ensure the exact moment systolic pressure returns.

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