What is the interpretation and management guide for patients with normal vs abnormal Ankle-Brachial Index (ABI) values?

Interpretation and Management Guide for Ankle-Brachial Index (ABI)

An ABI ≤0.90 confirms the diagnosis of peripheral artery disease (PAD) and requires immediate initiation of guideline-directed medical therapy, while values between 0.91-0.99 are borderline and warrant additional testing, particularly exercise ABI testing. 1, 2

ABI Classification and Interpretation

ABI Value	Classification	Interpretation	Next Steps
≤0.90	Abnormal	PAD confirmed	Initiate treatment, risk assessment
0.91-0.99	Borderline	Possible PAD	Exercise ABI testing
1.00-1.40	Normal	No significant PAD	If symptomatic, exercise ABI or other tests
>1.40	Noncompressible	Medial arterial calcification	Toe-brachial index (TBI)

Diagnostic Algorithm for Suspected PAD

1. Initial Assessment: Perform resting ABI with Doppler ultrasound technique

   • Use formula: ABI = Highest ankle systolic pressure / Highest brachial systolic pressure 2
   • Measure both arms to identify the higher systolic pressure 2

2. Interpret Results:

   • ABI ≤0.90: PAD diagnosis confirmed 1, 2
   • ABI 0.91-0.99: Borderline, proceed to exercise testing 1, 2
   • ABI 1.00-1.40 with symptoms: Proceed to exercise testing 1, 2
   • ABI >1.40: Noncompressible vessels, perform TBI 1, 2

3. Exercise ABI Testing:

   • Indicated when resting ABI is normal or borderline but clinical suspicion remains 1, 2
   • A post-exercise ankle pressure decrease >30 mmHg or ABI decrease >20% is diagnostic for PAD 1, 2
   • Significantly increases diagnostic sensitivity in symptomatic patients with normal resting ABI 3

Management Based on ABI Results

For Abnormal ABI (≤0.90):

• Initiate guideline-directed medical therapy 2:

  • Antiplatelet therapy
  • High-intensity statin therapy
  • Blood pressure control
  • Diabetes management
  • Smoking cessation

• Consider anatomical assessment if revascularization is being considered 1, 2:

  • Duplex ultrasound (first-line imaging)
  • CT angiography (CTA) or MR angiography (MRA)

For Borderline ABI (0.91-0.99):

• Perform exercise ABI testing 1, 2
• If exercise testing is positive, manage as abnormal ABI
• If negative but symptoms persist, consider alternative diagnoses

For Normal ABI (1.00-1.40) with Symptoms:

• Perform exercise ABI testing 1, 2
• Up to 31% of patients with normal resting ABI but symptoms will have abnormal post-exercise ABI 3
• Consider alternative causes of leg pain if all tests are normal

For Noncompressible Vessels (ABI >1.40):

• Perform toe-brachial index (TBI) 1, 2
• Consider pulse volume recordings or Doppler waveform analysis 2
• Particularly important in diabetic patients who frequently have arterial calcification 4

Special Considerations

Diabetic Patients:

• ABI has lower sensitivity in diabetic patients with complications 4
• Both ABI ≤0.90 and >1.30 indicate increased cardiovascular risk 4
• Consider TBI when there is clinical peripheral neuropathy or high risk of arterial calcification 4
• Patients with low TBI but normal ABI have similar comorbidity profiles to those with low ABI 5

Cardiovascular Risk Assessment:

• Abnormal ABI (≤0.90) identifies patients with approximately twice the risk of myocardial infarction and cardiovascular death 2, 6
• Even asymptomatic PAD in diabetic patients with ABI ≤0.90 is associated with higher risk of coronary or cerebrovascular morbidity and mortality 6

Common Pitfalls to Avoid

1. Relying solely on resting ABI in symptomatic patients

   • Nearly half of patients referred for suspected PAD may have normal resting ABI 3
   • Exercise ABI testing is essential when clinical suspicion remains despite normal resting ABI

2. Overlooking noncompressible vessels

   • ABI >1.40 does not indicate absence of disease but rather arterial calcification 1, 2
   • Always proceed to TBI in these cases

3. Using inappropriate technique

   • Using the higher ankle pressure provides better specificity (93-99%) but lower sensitivity (66-79%) 2
   • For diagnostic purposes in symptomatic patients, using the higher ankle pressure is recommended to avoid overdiagnosis 1

4. Neglecting post-exercise testing

   • Post-exercise ABI can detect PAD in 31% of symptomatic patients with normal resting ABI 3
   • Consider a post-exercise ankle pressure decrease >30 mmHg or ABI decrease >20% as diagnostic for PAD 1

5. Overreliance on ABI in diabetic patients

   • Both the resting and post-exercise ABI have lower diagnostic accuracy in diabetic patients 7
   • Consider additional vascular testing in diabetic patients 4, 7

By following this structured approach to ABI interpretation and management, clinicians can effectively diagnose PAD and implement appropriate treatment strategies to reduce cardiovascular morbidity and mortality.