What is the purpose and interpretation of the ankle brachial index (ABI) in diagnosing peripheral artery disease (PAD)?

Ankle Brachial Index: Purpose and Interpretation in Diagnosing Peripheral Artery Disease

The ankle brachial index (ABI) is the primary non-invasive diagnostic test for peripheral artery disease (PAD), with an ABI ≤0.90 confirming PAD diagnosis with high specificity (83-99%) but variable sensitivity (69-84%), serving as both a diagnostic tool and a powerful predictor of cardiovascular morbidity and mortality. 1

Definition and Measurement Technique

The ABI is calculated by dividing the highest systolic pressure in the ankle by the highest systolic brachial pressure:

• Measurement method: Doppler ultrasound technique is preferred over oscillometric or palpation methods for greater accuracy 2
• Calculation formula: ABI = Highest ankle systolic pressure / Highest brachial systolic pressure 1

Interpretation of ABI Values

According to the American College of Cardiology guidelines, ABI results are categorized as:

Category	ABI Value	Interpretation
Normal	1.00-1.40	No significant PAD
Borderline	0.91-0.99	Possible mild PAD
Abnormal	≤0.90	Diagnostic of PAD
Noncompressible vessels	>1.40	Arterial calcification (common in diabetes and CKD) [1]

Diagnostic Accuracy

The diagnostic performance of ABI varies based on:

• Method used: Doppler method shows higher accuracy (area under ROC curve 0.87-0.95) compared to oscillometric method (0.80-0.93) 2
• Ankle pressure selection: Using the higher ankle pressure provides better specificity (93-99%) but lower sensitivity (66-79%), while using the lower ankle pressure offers better sensitivity but lower specificity 2
• Overall accuracy: Pooled sensitivity of 75% and specificity of 86% for detecting significant arterial stenosis (≥50%) 3

Clinical Applications

1. Primary diagnosis of PAD:

   • First-line non-invasive test for PAD diagnosis 1
   • Particularly valuable in patients with risk factors (smoking, diabetes, hypertension, dyslipidemia) 1

2. Cardiovascular risk assessment:

   • An abnormal ABI (≤0.90) identifies patients with approximately twice the risk of myocardial infarction and cardiovascular death 1, 4
   • Serves as an independent predictor for cardiovascular mortality 4

3. Disease severity stratification:

   • Lower ABI values correlate with more severe disease and worse prognosis 2
   • Values <0.30 indicate critical limb ischemia with significantly higher cardiovascular risk 4

Diagnostic Approach Algorithm

1. Initial test: Resting ABI with Doppler ultrasound 1
2. If ABI ≤0.90: PAD diagnosis confirmed 1
3. If ABI is borderline (0.91-0.99) or normal but clinical suspicion remains: Perform exercise treadmill ABI testing 1
4. If ABI >1.40 (noncompressible vessels): Perform toe pressure/toe-brachial index (TBI) with waveforms 1

Important Considerations and Pitfalls

• Ankle pressure selection: Using the lower ankle pressure (LABI method) has higher sensitivity and overall accuracy for detecting PAD compared to using the higher ankle pressure (HABI method), especially in patients with diabetes and below-knee PAD 5

• Noncompressible vessels: In patients with diabetes or chronic kidney disease, arterial calcification may falsely elevate ABI values (>1.40), requiring alternative tests like toe-brachial index 1

• Borderline values: ABI values between 0.91-0.99 should be considered borderline and may warrant additional testing, especially with clinical suspicion 2, 1

• Pretest probability: The optimal ABI cutoff may be influenced by population characteristics and disease prevalence 2

• Serial measurements: For monitoring disease progression, consistent methodology should be used 2

Despite its utility, ABI remains underutilized in clinical practice 6. The U.S. Preventive Services Task Force concluded that evidence is insufficient to assess the balance of benefits and harms of screening for PAD with ABI in asymptomatic adults 2, but the test is clearly indicated for diagnosis in symptomatic patients or those with risk factors for PAD.