How do I calculate the ankle‑brachial index and interpret its values to decide whether compression stockings are appropriate?

How to Calculate and Interpret the Ankle-Brachial Index

Calculate the ABI by dividing the highest ankle systolic pressure (posterior tibial or dorsalis pedis) by the highest brachial systolic pressure from either arm; use the higher ankle pressure for diagnostic purposes to maximize specificity and avoid overdiagnosis of peripheral arterial disease. 1, 2

Patient Preparation

• Position the patient supine with head and heels fully supported for 5–10 minutes before measurement in a room at 19–22°C (66–72°F). 2
• Ensure the patient has not smoked for at least 2 hours before measurement, as smoking decreases ankle pressures and affects ABI results. 2
• Use an 8–10 MHz Doppler ultrasound probe with Doppler gel and appropriately sized blood pressure cuffs (width ≥40% of limb circumference). 2

Measurement Sequence

• Follow a standardized counterclockwise sequence: right arm → right posterior tibial (PT) → right dorsalis pedis (DP) → left PT → left DP → left arm. 1, 2
• Repeat the first arm measurement at the end of the sequence and average both readings unless they differ by >10 mm Hg; if the difference exceeds 10 mm Hg, discard the first measurement and use only the second. 1, 2

Calculation Method

• For diagnostic purposes (confirming PAD): Use the higher of the two ankle pressures (PT or DP) as the numerator to achieve specificity of 0.99 versus 0.93 and minimize false-positive diagnoses. 1, 2
• For cardiovascular risk assessment: Use the lower of the two ankle pressures as the numerator to achieve sensitivity of 0.89 versus 0.66 and identify more at-risk individuals. 1, 2
• Always use the highest brachial systolic pressure from either arm as the denominator. 1, 2

Example Calculation

If right arm = 130 mm Hg, left arm = 125 mm Hg, right PT = 120 mm Hg, right DP = 115 mm Hg:

• Diagnostic ABI = 120 (higher ankle) ÷ 130 (higher arm) = 0.92

Interpretation for Compression Therapy Decisions

ABI Value	Interpretation	Compression Therapy Decision
≤0.40	Severe PAD	Absolute contraindication; urgent vascular referral required [3]
0.41–0.59	Moderate PAD	Generally contraindicated; vascular assessment mandatory [3]
0.60–0.80	Mild PAD	Modified compression only with close monitoring; vascular referral needed [3]
0.81–0.90	Borderline PAD	Caution advised; consider vascular consultation before compression [3]
0.91–1.40	Normal	Safe for standard compression therapy [2,3]
>1.40	Non-compressible arteries	ABI invalid; obtain toe-brachial index before compression [2,4]

Critical Decision Points for Compression

• ABI <0.50 is an absolute contraindication to compression therapy due to high risk of tissue necrosis and limb loss. 3
• ABI >0.80 generally permits standard compression, but any abnormal value warrants vascular assessment before initiating therapy. 3
• Never rely solely on palpable pulses to assess arterial circulation; formal ABI measurement is mandatory before compression. 3

Special Populations Requiring Alternative Testing

When ABI >1.40 (Non-Compressible Vessels)

• Medial arterial calcification (Mönckeberg's sclerosis) makes the ABI unreliable by artificially elevating readings and masking true PAD. 2, 4
• Obtain toe-brachial index (TBI) instead: normal TBI ≥0.70, abnormal TBI <0.70. 2, 4
• This scenario is common in diabetes mellitus, chronic kidney disease, and end-stage renal disease. 2, 4

Diabetes and Chronic Kidney Disease

• Obtain TBI regardless of ABI value in diabetic patients, as arterial calcification may "normalize" a truly abnormal ABI even when <1.40. 4
• TBI ≥0.75 reliably excludes PAD in diabetic patients, whereas ABI may be falsely elevated. 4

Symptomatic Patients with Normal Resting ABI

• When clinical suspicion persists despite ABI 0.91–1.40, perform post-exercise ABI; a drop ≥0.15 indicates at least moderate arterial occlusive disease. 3
• Failure to perform post-exercise testing in symptomatic patients leads to missed diagnoses. 3

Common Pitfalls to Avoid

• Applying compression when ABI <0.50 can cause tissue necrosis and limb loss. 3
• Failing to perform ABI before compression in patients with lower extremity wounds leads to preventable adverse outcomes. 3
• Relying on a single measurement when values are borderline (0.91–1.00); repeat measurements improve accuracy. 2
• Using the lower ankle pressure for diagnostic confirmation increases false positives (sensitivity 0.89 but specificity only 0.93 versus 0.99 with higher pressure). 1
• Ignoring absent ankle artery signals; when one ankle artery is absent but the other shows normal ABI, perform duplex ultrasound to rule out PAD. 1

Training Requirements

• ABI may be performed by physicians, nurses, vascular technicians, or allied health professionals who have completed both didactic and hands-on training in vascular anatomy, PAD presentation, and Doppler technique. 2
• Trainees must demonstrate reproducible, independent performance in both healthy individuals and PAD patients before performing unsupervised testing. 2