Next Best Diagnostic Test for Arterial Ischemia/PAD Symptoms
The ankle-brachial index (ABI) is the next best diagnostic test for symptoms of arterial ischemia or peripheral arterial disease. 1
Why ABI is the First-Line Test
ABI is recommended as the initial non-invasive diagnostic test to confirm lower-limb decreased perfusion status in all patients with suspected PAD, with sensitivity of 68-84% and specificity of 84-99%. 1
The test is simple, inexpensive, widely available, and can be performed in office-based settings using only a blood pressure cuff and handheld Doppler device. 1
An ABI ≤0.90 confirms the diagnosis of PAD, while values between 0.91-0.99 are borderline, 1.00-1.40 are normal, and >1.40 indicate noncompressible arteries requiring alternative testing. 1
When to Proceed to US Doppler
Duplex ultrasound is recommended as the first-line imaging method only after ABI screening has been performed, not as the initial diagnostic test. 1, 2
US Doppler should be used when anatomic characterization of PAD lesions is needed, particularly when revascularization is being considered in symptomatic patients. 1
Duplex ultrasound has 85-90% sensitivity and >95% specificity for detecting stenosis >50%, but it is reserved for patients who have already been diagnosed with PAD via ABI and require anatomic localization. 3
When CT Angiography is Appropriate
CT angiography should not be performed as an initial diagnostic test for suspected PAD. 1, 3
CTA is recommended as adjuvant imaging in symptomatic patients with aorto-iliac or multisegmental/complex disease when revascularization is being planned. 1
Anatomic imaging with CTA (or MRA) is reserved for highly symptomatic patients in whom the diagnosis has already been established and intervention is being considered. 1
Special Circumstances Requiring Modified Approach
If the initial ABI is >1.40 (noncompressible arteries), which occurs commonly in patients with diabetes or chronic kidney disease due to medial arterial calcification:
- Measure toe-brachial index (TBI) with waveforms as the next test, with TBI <0.70 considered abnormal. 1
If the resting ABI is normal (>0.90) but clinical suspicion remains high due to exertional leg symptoms:
- Perform post-exercise ABI testing, preferably by treadmill, with a post-exercise ABI decrease of >20% serving as a diagnostic criterion for PAD. 1
If chronic limb-threatening ischemia (CLTI) is suspected with rest pain, non-healing wounds, or gangrene:
- Additional perfusion measures beyond ABI should be obtained, including toe pressure (abnormal <30 mmHg), TcPO2 (abnormal <30 mmHg), or skin perfusion pressure. 1
Common Pitfalls to Avoid
Do not skip ABI and proceed directly to imaging studies unless there is an urgent need for revascularization planning in a patient with obvious critical limb ischemia. 1, 3
Do not rely on ABI alone in diabetic patients or those with renal failure, as medial arterial calcification can produce falsely elevated values; measure TBI in these populations even if resting ABI appears normal. 1
Do not order CT angiography for asymptomatic PAD or for patients who are not revascularization candidates, as anatomic imaging is not indicated without a plan for intervention. 1, 3
The sensitivity of ABI can be as low as 15% in elderly individuals and patients with diabetes, so maintain clinical suspicion and proceed to TBI or exercise testing when the clinical picture doesn't match a normal resting ABI. 4