What is the next step in management for a patient with an Ankle-Brachial Index (ABI) result showing a potential arterial obstruction in the right leg, with ABI values of 110 in both arms and 120 in the left leg, but 100 in the right leg?

ABI Calculation and Interpretation

Your calculated ABI values are normal (right leg ABI = 0.91, left leg ABI = 1.09), indicating no significant peripheral arterial disease at rest, and no immediate intervention is required beyond standard cardiovascular risk factor management.

How to Calculate ABI Correctly

The ABI is calculated by dividing the ankle systolic pressure by the brachial systolic pressure. 1 Here's the proper method:

• Use the higher brachial pressure as the denominator: Your higher arm pressure is 110 mmHg (both arms equal) 1
• Use the higher ankle pressure for each leg as the numerator: For diagnostic purposes, use the higher of the dorsalis pedis or posterior tibial pressure 2
• Calculate for each leg separately:
  • Right leg ABI = 100/110 = 0.91
  • Left leg ABI = 120/110 = 1.09

Interpretation of Your Results

Right leg ABI of 0.91 falls in the borderline range (0.91-0.99), while left leg ABI of 1.09 is normal (1.00-1.40). 1

What Borderline ABI Means

• A borderline ABI (0.91-0.99) does not definitively diagnose PAD but warrants clinical correlation 1
• The patient may have early or mild arterial disease that is not yet hemodynamically significant 3
• If the patient has exertional leg symptoms (claudication), exercise treadmill ABI testing is indicated to unmask arterial insufficiency that may not be apparent at rest 1, 4

Next Steps in Management

If Patient is Asymptomatic

No further vascular testing is needed at this time. 1 Focus on:

• Comprehensive cardiovascular risk factor assessment and modification 1
• Measure blood pressure in both arms to confirm no subclavian stenosis (already done - both 110 mmHg) 1
• Standard preventive care for atherosclerotic disease 1

If Patient Has Exertional Leg Symptoms

Exercise treadmill ABI testing is recommended (Class I, Level B-NR). 1 This is critical because:

• Nearly half of symptomatic patients referred for PAD evaluation have normal resting ABI 4
• Among patients with normal resting ABI who undergo exercise testing, 31% demonstrate a significant post-exercise ABI drop indicating PAD 4
• A post-exercise ankle pressure decrease >30 mmHg or ABI decrease >20% is diagnostic for PAD 1

If Patient Has Non-Healing Wounds or Rest Pain

Additional perfusion assessment is warranted even with borderline ABI. 1 Consider:

• Toe-brachial index (TBI) with waveforms 1
• Transcutaneous oxygen pressure (TcPO2) 1
• Skin perfusion pressure (SPP) 1

Important Clinical Caveats

The Inter-Arm Pressure Difference

Your patient has equal bilateral arm pressures (110 mmHg), which is reassuring. An inter-arm difference >15-20 mmHg would suggest subclavian or innominate artery stenosis and would require using the higher arm pressure for ABI calculation. 1, 2

Alternative ABI Calculation Methods

For cardiovascular risk stratification (not diagnosis), some experts recommend using the lower ankle pressure instead of the higher one. 3, 5 This approach:

• Would give your patient a right leg ABI of 100/110 = 0.91 (same result if only one ankle artery was measured)
• Identifies more patients at cardiovascular risk who might be missed using the higher ankle pressure 5
• In one study, patients with ABI ≥0.9 by standard calculation but <0.9 by modified calculation (using lower ankle pressure) had comparable cardiovascular event rates to those with frank PAD 5

When Resting ABI May Be Falsely Normal

Be aware that resting ABI has limitations:

• Sensitivity ranges from only 68-84% for detecting significant arterial stenosis 1
• May miss moderate stenoses that become hemodynamically significant only with exercise 4
• This is why exercise ABI testing is crucial in symptomatic patients with normal or borderline resting values 1, 4

Special Populations

If your patient has diabetes or chronic kidney disease, be alert for:

• Medial arterial calcification causing falsely elevated or non-compressible vessels (ABI >1.40) 1
• In such cases, toe-brachial index should be measured instead (Class I, Level B-NR) 1
• TBI ≤0.70 is abnormal and confirms PAD 2

Documentation and Follow-Up

Report ABI values to two decimal places using standardized categories: 1

• Right leg: ABI = 0.91 (borderline)
• Left leg: ABI = 1.09 (normal)

For patients with borderline ABI, clinical context determines next steps - specifically whether exertional leg symptoms are present. 1, 3