What does a level 2 normal ankle‑brachial index (ABI) indicate and how should it be managed in an otherwise healthy adult?

Normal Ankle-Brachial Index: Definition and Clinical Implications

A normal ankle-brachial index (ABI) ranges from 0.91 to 1.40, indicating adequate arterial blood flow to the lower extremities and absence of significant peripheral arterial disease. 1

Understanding Normal ABI Values

The ABI is calculated as the ratio of ankle systolic pressure to brachial systolic pressure, measured after 10 minutes of rest in the supine position. 1

Normal range interpretation:

• Optimal range: 1.11 to 1.40 – associated with lowest cardiovascular risk 2
• Borderline normal: 0.91 to 1.00 – requires clinical context and may warrant further evaluation 2
• Values 0.91-1.40 are considered normal per American Heart Association and American College of Cardiology guidelines 1, 2

Clinical Significance of Normal ABI

A normal ABI indicates:

• Adequate arterial perfusion to lower extremities 2
• Lower risk of cardiovascular events and mortality compared to abnormal values 2
• Absence of hemodynamically significant atherosclerotic disease in the lower extremity arteries 2

The relationship between ABI and mortality follows a reverse J-shaped curve, with the lowest cardiovascular risk occurring in the 1.11-1.40 range. 2

Important Caveats and Limitations

A normal resting ABI does not completely exclude peripheral arterial disease in symptomatic patients. 2

Key limitations to recognize:

• Post-exercise ABI may be abnormal despite normal resting values – a drop of ≥0.15 after exercise suggests at least moderate arterial occlusive disease 1, 2
• ABI cannot localize disease or distinguish single-level from multilevel disease 1
• In patients with diabetes or chronic renal insufficiency, arterial calcification may produce falsely elevated ABI values (>1.3), making the test unreliable 1

Management Approach for Normal ABI

For asymptomatic patients with normal ABI (0.91-1.40):

• No immediate vascular intervention required 2
• Standard cardiovascular risk factor modification 2

For symptomatic patients with normal resting ABI:

• Perform post-exercise ABI testing when clinical suspicion of PAD remains despite normal resting values 1, 2
• Consider duplex ultrasound or other imaging if symptoms persist 2

For borderline values (0.91-1.00) with cardiovascular risk factors:

• Perform annual bilateral ABI measurements to monitor for disease progression 2
• Initiate aggressive cardiovascular risk factor modification including high-intensity statin therapy, smoking cessation, blood pressure control, and glycemic optimization in diabetics 2
• If follow-up ABI drops to ≤0.90, immediate vascular imaging is indicated 2

Special Populations Requiring Alternative Testing

When ABI is >1.40 (non-compressible vessels):

• Measure toe-brachial index (TBI) as the preferred alternative, with TBI >0.70 considered normal 2, 3
• This scenario is common in patients with long-standing diabetes mellitus and chronic renal insufficiency 1

For patients with diabetes regardless of ABI value:

• Consider TBI measurement even when ABI appears normal, as arterial calcification may mask underlying PAD 3

Common Pitfalls to Avoid

• Relying on a single measurement when values are borderline – repeat measurements improve accuracy 2, 3
• Measuring only one limb – PAD may not progress symmetrically, so bilateral measurements are essential 2, 4
• Failing to consider post-exercise testing in symptomatic patients with normal resting ABI 1, 2
• Not recognizing that palpable pulses alone are insufficient to exclude PAD 2