How can an Ankle-Brachial Index (ABI) test be normal with significant blood pressure discrepancies between the left and right legs?

Understanding Normal ABI with Blood Pressure Discrepancies Between Legs

Your ABI test was correctly interpreted as normal because the ABI calculation uses the higher ankle pressure from each leg divided by the higher brachial pressure, and both of your legs likely had ABI values >0.90, which is the threshold for normal. 1

Why Blood Pressure Differences Between Legs Don't Necessarily Indicate Abnormal ABI

Standard ABI Calculation Method

• The ABI is calculated separately for each leg using the higher of the two ankle pressures (posterior tibial or dorsalis pedis artery) divided by the higher brachial pressure from either arm 1, 2
• Each leg gets its own ABI value, and a 20 mmHg difference between corresponding levels (like your thigh pressures of 160 vs 180 mmHg) does not automatically make the test abnormal 1
• An ABI ≥0.90 is considered normal, and values between 0.91-1.40 indicate adequate arterial blood flow 1, 3

Why Side-to-Side Differences Are Common and Often Normal

• Peripheral arterial disease may be unilateral or asymmetric, meaning one leg can have more disease than the other while both still maintain adequate perfusion 1
• Segmental pressure measurements naturally show variations between limbs due to anatomical differences, measurement technique, and physiological variation 1
• A pressure gradient of 20 mmHg between adjacent levels on the same leg suggests hemodynamically significant stenosis, but differences between legs at the same level are interpreted differently 1

When Lower Pressures on One Side Become Clinically Significant

The Key Threshold: ABI <0.90

• The critical question is whether your lower-pressure leg had an ABI <0.90, not whether there was a difference between legs 1
• If your left leg had thigh pressure of 160 mmHg and ankle pressure that still yielded an ABI >0.90, the test is appropriately reported as normal 1, 3
• Using your higher brachial pressure as the denominator, even the leg with lower pressures may have maintained an adequate ABI 1, 2

Segmental Pressure Interpretation

• Segmental pressures (thigh, calf, ankle) help localize disease anatomically but don't define the overall ABI result 1
• These measurements are particularly useful when planning revascularization, not necessarily for initial PAD diagnosis 1
• The 20 mmHg difference you observed between your thighs could reflect normal anatomical variation if both legs maintained adequate ankle pressures 1

Potential Limitations and When Further Testing Is Warranted

When Normal ABI May Miss Disease

• ABI has only 69-79% sensitivity for PAD, meaning it can miss disease in 21-31% of cases, particularly in patients with diabetes 1
• One study found ABI had only 72.3% sensitivity and 69.3% specificity when compared to duplex ultrasound, with false-negative results in 27.5% of limbs 4
• If you have symptoms suggestive of PAD (leg pain with walking that resolves with rest), a normal resting ABI doesn't completely exclude disease 1, 3

Additional Testing to Consider

• Post-exercise ABI testing should be performed if you have exertional leg symptoms despite normal resting ABI, looking for a drop >20% in ABI or >30 mmHg in ankle pressure after exercise 1
• Pulse volume recordings (PVR) or Doppler waveforms at the ankle can confirm concordance with ABI results and help identify discrepancies 1
• Duplex ultrasound provides anatomic detail with 85-90% sensitivity and >95% specificity for stenosis >50% if there's clinical suspicion despite normal ABI 2

Common Pitfalls in ABI Interpretation

What Clinicians Should Avoid

• Relying on a single borderline measurement (0.91-1.00) without repeat testing is a common error; the American Heart Association recommends repeat measurements in such cases 2
• Assuming symmetric disease progression when PAD commonly affects legs asymmetrically 1
• Not considering post-exercise testing when resting ABI is normal but clinical suspicion remains high 1, 3
• Failing to recognize that normal ABI doesn't exclude mild-to-moderate disease, particularly in early stages 4, 5

Was There an Error in Your Test?

Most Likely Scenario: No Error

• If both your legs had ABI values ≥0.90, the test was correctly reported as normal despite the pressure differences between legs 1, 3
• The side-to-side differences you observed are documented in segmental pressures but don't determine the final ABI interpretation 1
• Your vascular laboratory likely calculated each leg's ABI using the higher ankle pressure from that leg divided by your higher brachial pressure 1, 2

When to Request Clarification

• Ask your physician for the specific ABI values calculated for each leg (right ABI = X, left ABI = Y) to understand why the test was deemed normal 2
• If you have symptoms of claudication (leg pain with walking), request post-exercise ABI testing even with normal resting values 1
• Consider requesting pulse volume recordings or Doppler waveforms to assess the quality of blood flow at each level and confirm concordance with the ABI results 1