What are the steps and materials needed to perform Ankle-Brachial Index (ABI) and Toe-Brachial Index (TBI) tests?

How to Perform Ankle-Brachial Index (ABI) and Toe-Brachial Index (TBI)

Both ABI and TBI should be performed in a supine position after 5-10 minutes of rest in a room with constant temperature, using standardized protocols to ensure accurate diagnosis of peripheral arterial disease. 1, 2

Materials Needed

For ABI:

• Blood pressure cuffs (appropriate size: width should be at least 40% of limb circumference) 3
• Hand-held Doppler ultrasound device (8-10 MHz probe) 3
• Doppler gel 3
• Sphygmomanometer (manual or aneroid) 4
• Impermeable dressing for covering any open wounds 3

For TBI:

• Blood pressure cuffs for upper arms 1
• Small cuffs for toes 1
• Photoplethysmography probe (placed on distal pulp of first or second toe) 1
• Doppler device 1

Patient Preparation (Both Tests)

• Position patient supine with head and heels fully supported 1, 3
• Rest period: Allow 5-10 minutes of rest before measurement 1, 3
• Room temperature: Maintain constant temperature (19°C–22°C/66°F–72°F) 3
• No smoking: Patient should not have smoked for at least 2 hours before measurement 3
• Cover wounds: Apply impermeable dressing to any open wounds to prevent contamination 3

Step-by-Step ABI Procedure

Measurement Sequence:

Follow this exact order as recommended by the American Heart Association 3:

1. First arm (brachial artery)
2. First leg posterior tibial (PT) artery
3. First leg dorsalis pedis (DP) artery
4. Second leg PT artery
5. Second leg DP artery
6. Second arm (brachial artery)

Detailed Steps:

Brachial Pressure Measurement:

• Apply appropriately sized cuff to upper arm 2
• Locate brachial artery with Doppler probe using gel 3
• Inflate cuff 20-30 mmHg above point where signal disappears 4
• Slowly deflate and record systolic pressure when signal returns 4
• Repeat on opposite arm 2
• If first arm SBP exceeds second arm by >10 mmHg, remeasure first arm and disregard initial reading 3

Ankle Pressure Measurement:

• Apply cuff just above ankle 1
• Avoid placing cuff over recent bypass grafts (risk of thrombosis) 3
• Locate PT artery (behind medial malleolus) with Doppler probe 2
• Inflate cuff 20-30 mmHg above signal disappearance 4
• Slowly deflate and record systolic pressure 2
• Repeat for DP artery (dorsum of foot) 2
• Measure both arteries in both legs 2, 3

ABI Calculation

For diagnostic purposes (preferred method to minimize overdiagnosis) 2, 3:

• ABI = Higher ankle pressure (PT or DP) / Higher brachial pressure 2

For cardiovascular risk assessment (identifies more at-risk individuals) 3:

• ABI = Lower ankle pressure / Higher brachial pressure 3

Step-by-Step TBI Procedure

When to perform: TBI is indicated when ABI >1.40 (non-compressible arteries) or in patients with diabetes/chronic kidney disease where medial calcification is suspected 1, 2, 5

Detailed Steps:

1. Measure brachial pressures as described above 1
2. Apply small cuff to base of great toe or second toe 1
3. Place photoplethysmography probe on distal pulp of toe 1
4. Inflate cuff until signal disappears 1
5. Slowly deflate and record systolic pressure when signal returns 1
6. Repeat on opposite foot 1

TBI Calculation:

• TBI = Toe systolic pressure / Higher brachial systolic pressure 1

Results Interpretation

ABI Values:

• Normal: 0.91-1.40 (optimal: 1.11-1.40) 2, 6
• Borderline: 0.90-1.00 1, 6
• Abnormal (PAD confirmed): ≤0.90 1, 2, 5
• Non-compressible arteries: >1.40 1, 2, 5

TBI Values:

• Normal: >0.70 1, 2
• Abnormal: ≤0.70 1, 2

Critical Limb-Threatening Ischemia (CLTI) Criteria:

• Ankle pressure: <50 mmHg 1
• Toe pressure: <30 mmHg 1

Important Nuances and Pitfalls

Accuracy Considerations:

• Sensitivity: ABI has 68-84% sensitivity and 84-99% specificity for PAD diagnosis 1
• Reduced sensitivity in patients with diabetes or end-stage chronic kidney disease due to arterial calcification 2, 5
• Doppler method: Manual Doppler by untrained personnel has lower specificity (56%) compared to oscillometric automated methods (89%) 7
• Both oscillometric and Doppler methods show good concordance when performed correctly 1

Common Pitfalls to Avoid:

• Not measuring both PT and DP arteries: Could miss disease affecting only one vessel 8
• Single measurement in borderline cases: Repeat measurements improve accuracy; at least 2 sets should be performed for research or when values are 0.91-1.00 4, 3
• Relying on palpable pulses alone: Insufficient for assessing arterial circulation 6
• Ignoring arm pressure discrepancy: Always remeasure if >10 mmHg difference between arms 3
• Not considering alternative tests: When ABI >1.40 with clinical suspicion of PAD, must perform TBI 1, 2, 5

When Normal Resting ABI Doesn't Rule Out PAD:

• Post-exercise ABI should be considered when resting ABI is normal (>0.90) but clinical suspicion remains 1, 2
• Exercise protocol: Treadmill at 3 km/h, 10% slope (Strandness protocol) 5
• Diagnostic criteria: Post-exercise ankle SBP decrease >30 mmHg OR post-exercise ABI decrease >20% confirms PAD 5
• Measure post-exercise ABI at 1 minute after cessation of exercise, starting with symptomatic leg 1

Operator Training Requirements:

• Trained physicians have better reproducibility than inexperienced operators 1
• Training must include: Understanding of vascular anatomy, physiology, clinical presentation of PAD, Doppler device function, demonstration of each step, and ability to show reproducible results 4, 3
• Competence verification: Trainees must independently demonstrate correct performance in both healthy individuals and PAD patients 4

Special Populations:

• Diabetes/CKD patients: Lower sensitivity due to medial calcification; proceed directly to TBI if ABI >1.40 2, 5
• Elderly patients: Sensitivity particularly reduced; consider TBI as first-line test 9
• Bilateral disease: Always measure both limbs as PAD may not progress symmetrically 3

Adjunctive Tests:

• Doppler waveforms and pulse volume recording (PVR) at ankle can confirm concordance with ABI and suggest non-compressible arteries when discordant 2
• Duplex ultrasound provides anatomic and hemodynamic information with 85-90% sensitivity and >95% specificity for stenosis >50% 5

Time Requirements:

• Automated ABI: Average 5.8 minutes 10
• Manual Doppler ABI: Average 9.3 minutes 10
• Comprehensive research protocol: Should include duplicate measurements with averaging for increased accuracy 4