How do I write an order for an ankle‑brachial index (ABI) study of both lower extremities, including patient age, sex, and relevant vascular risk factors?

How to Write an ABI Order for Lower Extremities

Order a bilateral resting ankle-brachial index (ABI) with toe-brachial index (TBI) for both lower extremities, specifying measurement of bilateral brachial pressures, bilateral dorsalis pedis and posterior tibial artery pressures with Doppler waveform analysis, and bilateral great toe pressures—this comprehensive approach is essential because ABI alone has only 65-72% sensitivity for peripheral artery disease (PAD) and may be falsely elevated in patients with diabetes or arterial calcification. 1, 2

Essential Components of the ABI Order

Basic Order Structure

• Order name: "Bilateral ankle-brachial index with toe-brachial index and Doppler waveform analysis" 1
• Include patient demographics: Age, sex, and all relevant vascular risk factors (smoking history, diabetes, hypertension, hyperlipidemia, chronic kidney disease) must be documented in the order to guide interpretation 1, 3

Specific Measurements to Request

Brachial pressures:

• Measure bilateral brachial systolic pressures in a standardized sequence: first arm measured initially, then repeated at the end of the measurement series 1
• If the two arm measurements differ by >10 mmHg, use the higher value as the denominator for all ABI calculations 1

Ankle pressures:

• Measure bilateral dorsalis pedis (DP) and posterior tibial (PT) artery systolic pressures using continuous-wave Doppler 1
• Document Doppler waveform characteristics at each ankle artery (triphasic, biphasic, or monophasic) 1, 3
• Critical technical point: Use the higher of the DP or PT pressure from each leg as the numerator when calculating ABI—this maximizes specificity and reduces false-positives, though some evidence suggests using the lower pressure identifies more at-risk patients 1, 4, 5

Toe pressures (mandatory in high-risk patients):

• Measure bilateral great toe systolic pressures using photoplethysmography or Doppler 3, 6
• This is non-negotiable in patients with diabetes, chronic kidney disease, age >70 years, or any suspicion of arterial calcification 1, 3

Calculation and Interpretation Specifications

ABI Calculation Formula

• ABI = (higher ankle pressure of DP or PT) ÷ (higher brachial pressure) 1, 3
• Calculate separately for each leg and report both values 1

TBI Calculation Formula

• TBI = (toe systolic pressure) ÷ (higher brachial pressure) 3, 6

Interpretation Thresholds to Specify in Order

• ABI ≤0.90: Confirms PAD diagnosis 1
• ABI 0.91-0.99: Borderline; requires additional testing (exercise ABI or TBI) 1
• ABI 1.00-1.40: Traditionally normal, but unreliable in diabetes—proceed to TBI regardless 1, 3
• ABI >1.40: Indicates noncompressible vessels from medial arterial calcification; ABI is invalid and TBI is mandatory 1, 3
• TBI <0.70-0.75: Confirms significant PAD 3, 6

Patient Preparation Requirements

• Position patient supine for at least 10 minutes before any measurements to allow hemodynamic stabilization 1, 3
• Ensure room temperature is warm to prevent arterial vasoconstriction that could yield falsely low pressures 6
• Patient should avoid caffeine, smoking, and exercise for at least 2 hours prior to testing 1

When to Add Exercise ABI Testing

Include post-exercise ABI measurement if:

• Patient has exertional leg symptoms (claudication) but resting ABI is normal (0.91-1.40) 1
• Clinical suspicion for PAD remains high despite normal resting ABI 1
• Diagnostic criteria: Post-exercise ankle pressure decrease >30 mmHg or ABI decrease >20% confirms PAD 1

Critical Pitfalls to Avoid

Never Rely on ABI Alone in High-Risk Populations

• In diabetic patients: ABI has falsely normal or elevated readings in up to 50% of cases due to medial arterial calcification (Mönckeberg sclerosis), making TBI mandatory regardless of ABI result 3, 6
• In patients with chronic kidney disease or age >70: Arterial calcification is highly prevalent and ABI sensitivity drops significantly 1, 3

Never Accept "Normal" ABI as Excluding PAD

• ABI is normal in 29% of patients with chronic limb-threatening ischemia 6
• Concordance between ABI and toe pressure is poor—only 58% of patients with abnormal toe pressures have abnormal ABI 6
• Research demonstrates ABI sensitivity is only 65-72% when compared to duplex ultrasound or angiography as reference standards 2, 4

Never Use Single-Leg Measurement

• Always measure both legs—PAD may be unilateral or asymmetric, and using only one leg misses patients at high cardiovascular risk 1

Sample Order Template

"Bilateral ankle-brachial index with toe-brachial index and Doppler waveform analysis:

• Measure bilateral brachial systolic pressures (repeat first arm at end of series)
• Measure bilateral dorsalis pedis and posterior tibial artery systolic pressures with Doppler
• Document Doppler waveform characteristics at each ankle artery
• Measure bilateral great toe systolic pressures
• Calculate ABI for each leg using higher ankle pressure ÷ higher brachial pressure
• Calculate TBI for each leg using toe pressure ÷ higher brachial pressure
• Patient preparation: supine position for 10 minutes before testing in warm room

Patient demographics: [Age], [Sex]
Vascular risk factors: [Diabetes Y/N], [Smoking history], [Hypertension Y/N], [Hyperlipidemia Y/N], [CKD Y/N]
Clinical indication: [Exertional leg symptoms / nonhealing wound / screening in high-risk patient / etc.]"

When to Proceed to Advanced Imaging

• If ABI ≤0.90 or TBI <0.70 and patient has symptoms requiring intervention, proceed directly to duplex ultrasound, CT angiography, or MR angiography for anatomic localization 1, 3
• If ankle pressure <50 mmHg or ABI <0.5, urgent vascular surgery referral for revascularization consideration 3