How do I interpret Ankle-Brachial Index (ABI) results in an adult patient with a possible history of smoking, diabetes, or hypertension?

Interpreting Ankle-Brachial Index (ABI) Results

ABI results should be uniformly reported and interpreted using standardized thresholds: values ≤0.90 indicate peripheral artery disease (PAD), 0.91-0.99 are borderline and warrant further evaluation, 1.00-1.40 are normal, and >1.40 indicate noncompressible vessels requiring alternative testing. 1

Standard ABI Classification and Diagnostic Thresholds

The American College of Cardiology and American Heart Association establish the following interpretation framework 1:

• ABI ≤0.90: Diagnostic for PAD 1
• ABI 0.91-0.99: Borderline, indicating subclinical PAD and increased cardiovascular risk 1
• ABI 1.00-1.40: Normal range, with optimal values between 1.11-1.40 1
• ABI >1.40: Noncompressible vessels due to arterial calcification (medial sclerosis) 1

Severity Stratification When ABI is Abnormal

For patients with ABI ≤0.90, further stratification guides management 2:

• Severe PAD: ABI <0.50 (high amputation risk, urgent vascular referral needed) 2
• Moderate PAD: ABI 0.50-0.69 2
• Mild PAD: ABI 0.70-0.89 2

Critical Interpretation Pitfalls in High-Risk Populations

Diabetic Patients and Arterial Calcification

Never rely on ABI alone in diabetic patients—arterial calcification causes falsely normal or elevated readings despite significant ischemia. 3, 4

• Medial arterial calcification (Mönckeberg sclerosis) is highly prevalent in diabetes and renders ABI unreliable 3, 4
• Up to 50% of patients with ABI >1.40 have coexisting occlusive PAD masked by calcification 3
• When ABI is >1.30-1.40 in diabetic patients, immediately obtain toe-brachial index (TBI); values <0.70-0.75 confirm PAD 1, 3

Patients with Smoking History or Hypertension

The resting ABI should be measured in all patients ≥50 years with smoking history or diabetes, and in all patients ≥65 years regardless of risk factors 1

When Normal or Borderline ABI Requires Additional Testing

Borderline ABI (0.91-0.99)

Patients with borderline ABI have significantly increased risk of functional decline and cardiovascular events 5:

• If exertional leg symptoms present: Proceed immediately to exercise treadmill ABI testing 3
• A post-exercise ankle pressure decrease >30 mmHg or ABI decrease ≥0.15-0.20 confirms PAD 3
• If asymptomatic: Implement aggressive cardiovascular risk reduction and annual surveillance 3

Normal ABI (1.00-1.40) with Persistent Clinical Suspicion

A normal resting ABI does not reliably exclude PAD in symptomatic patients, particularly those with isolated iliac disease. 3

• Exercise ABI testing is mandatory for patients with exertional non-joint-related leg symptoms and normal resting ABI 3
• In diabetic patients with normal ABI but foot symptoms, non-healing wounds, or absent pulses, obtain TBI immediately 3

Cardiovascular Risk Stratification Beyond PAD Diagnosis

The relationship between ABI and mortality follows a reverse J-shaped curve 1:

• Lowest cardiovascular risk: ABI 1.11-1.40 (reference range) 1
• Increased risk: ABI <0.90 or >1.40 1
• Any abnormal ABI (≤0.90 or >1.40) identifies patients requiring aggressive cardiovascular risk modification regardless of PAD symptoms 1, 2

Measurement Technique Requirements

Proper ABI measurement technique is essential for accurate interpretation 1:

• Patient must be supine with head and heels fully supported for 10 minutes before measurement 1
• Measure bilateral ankle systolic pressures and brachial pressures 1
• ABI is calculated as ankle systolic pressure divided by brachial systolic pressure 1
• Seated measurements yield ABI values approximately 0.35 higher than supine measurements and should not be used 1

Alternative Testing When ABI is Unreliable

Toe-Brachial Index (TBI)

TBI should be used when ABI is unreliable due to noncompressible vessels, particularly in patients with long-standing diabetes or advanced age 1:

• TBI <0.70-0.75: Confirms PAD 3, 6
• Digital arteries are rarely affected by medial calcification, making TBI more reliable than ABI in diabetic patients 3
• Studies show 20.5% of patients with suspected PAD have low TBI but normal ABI, representing significant underdiagnosis when ABI is used alone 6

Additional Vascular Testing

When ABI or TBI confirms PAD, or when clinical suspicion remains high 1:

• Leg segmental pressure measurements localize anatomic disease when revascularization planning is needed 1
• Doppler waveform analysis: triphasic waveforms strongly exclude PAD 3
• Duplex ultrasound, CTA, or MRA for anatomic localization before intervention 3, 2

Management Implications Based on ABI Results

ABI ≤0.90 (PAD Confirmed)

Implement comprehensive cardiovascular risk reduction 2:

• Antiplatelet therapy (aspirin or clopidogrel) 2
• High-intensity statin therapy with LDL-C target <55 mg/dL 2
• Smoking cessation support 1
• Blood pressure and glycemic control 2

ABI <0.50 or Ankle Pressure <50 mmHg

These values indicate critical limb-threatening ischemia requiring urgent vascular surgery referral for revascularization consideration 3, 2

ABI >1.40 (Noncompressible Vessels)

• Obtain TBI to assess for underlying PAD 1
• Recognize this finding itself indicates increased cardiovascular mortality risk independent of stenotic disease 1
• Implement aggressive cardiovascular risk reduction 3

Common Clinical Errors to Avoid

Never assume PAD is absent based solely on palpable pulses—even skilled examiners can detect pulses despite significant ischemia 3

Never delay vascular assessment in patients with non-healing wounds—objective testing with ABI and TBI is mandatory, as clinical examination sensitivity is too low to rule out PAD 3

Never use ABI alone for compression therapy decisions—ABI <0.50 is an absolute contraindication to compression, and ABI 0.50-0.80 requires modified compression protocols 7

Never interpret a single ABI measurement in isolation—always measure bilateral ABI, consider clinical context, and use the lower value for cardiovascular risk assessment 3