What does a normal ankle-brachial index (ABI) indicate?

Normal Ankle-Brachial Index Values and Their Significance

A normal ankle-brachial index (ABI) value ranges from 0.91 to 1.40, indicating absence of significant peripheral arterial disease (PAD) and lower cardiovascular risk compared to individuals with abnormal values. 1

Understanding ABI Values

• Normal ABI values fall between 0.91 and 1.40, with the optimal range being 1.11 to 1.40 1
• Values between 0.91 and 1.00 are considered borderline and should be interpreted with clinical context 1
• ABI ≤0.90 indicates peripheral arterial disease 1
• ABI >1.40 suggests non-compressible arteries, often due to arterial calcification 1

Clinical Significance of Normal ABI

• A normal ABI indicates adequate arterial blood flow to the lower extremities 1
• Normal values correlate with lower risk of cardiovascular events and mortality 1
• The relationship between ABI and mortality follows a reverse J-shaped curve, with lowest risk in the 1.11-1.40 range 1
• Normal ABI suggests absence of significant atherosclerotic disease in the lower extremities 1

Factors Affecting ABI Interpretation

• Clinical context must be considered when interpreting borderline values (0.91-1.00) 1
• For example, a 47-year-old woman with atypical calf pain, no cardiovascular disease history or risk factors, and an ABI of 0.91 has a low probability of PAD 1
• However, a man with classic intermittent claudication who smokes and has an ABI of 0.96 has a high probability of PAD despite the "borderline" value 1
• Body position affects ABI measurements - patients should be lying flat with head and heels fully supported 1

ABI Measurement Techniques

• ABI is calculated as the ratio of ankle systolic pressure to brachial systolic pressure 1
• Different methods exist for determining which ankle pressure to use:
  • Higher ankle pressure (more specific, fewer false positives) 1
  • Lower ankle pressure (more sensitive, fewer false negatives) 1
  • Average of posterior tibial and dorsalis pedis pressures 1
• For diagnostic purposes, using the higher ankle pressure is preferred to minimize overdiagnosis 1
• For cardiovascular risk assessment, using the lower ankle pressure identifies more individuals at risk 1

Clinical Applications

• ABI is a cost-effective screening tool for PAD in at-risk populations 2
• Normal ABI values are reassuring but do not completely exclude PAD in symptomatic patients 1
• When clinical suspicion remains despite normal resting ABI, post-exercise ABI or other imaging should be considered 1
• Normal ABI correlates with better long-term cardiovascular outcomes and lower all-cause mortality 3

Important Caveats

• ABI may be normal at rest but abnormal after exercise in some patients with PAD 1
• Post-exercise ABI should be considered when resting ABI is normal but clinical suspicion of PAD remains 1
• Serial ABI measurements may be more accurate than a single measurement 1
• ABI should be measured in both limbs as PAD may not progress symmetrically 1
• ABI has limitations in patients with diabetes or end-stage renal disease due to arterial calcification 1