What is the recommended management for a patient with a borderline ankle‑brachial index (ABI 0.91–0.99)?

Management of Borderline Ankle-Brachial Index (ABI 0.91–0.99)

Patients with borderline ABI (0.91–0.99) should undergo further diagnostic evaluation with exercise treadmill ABI testing if they have exertional leg symptoms, and should receive comprehensive cardiovascular risk factor management including statin therapy and blood pressure control, though the benefit of antiplatelet therapy specifically for this borderline range remains uncertain.

Diagnostic Approach

Initial Classification and Risk Stratification

A borderline ABI (0.91–0.99) represents an intermediate zone that signals increased cardiovascular risk but does not definitively confirm peripheral artery disease (PAD) 1. These patients are considered "borderline" in terms of cardiovascular risk and warrant further evaluation 1.

Key diagnostic steps:

• If the patient has exertional, non-joint-related leg symptoms: Proceed immediately to exercise treadmill ABI testing 2, 3. This is a Class I recommendation with strong evidence.

  • A post-exercise ankle pressure decrease >30 mmHg OR
  • A post-exercise ABI decrease >20% confirms PAD 1

• If the patient is asymptomatic: The borderline ABI itself indicates elevated cardiovascular risk requiring aggressive risk factor modification 1

Additional Testing Considerations

If exercise testing is not feasible or results are equivocal, consider:

• Duplex ultrasound to visualize arterial lesions 4, 3
• Toe-brachial index if there's concern for noncompressible vessels (though this is more relevant for ABI >1.40) 2, 3

Medical Management

Cardiovascular Risk Reduction (Mandatory)

All patients with borderline ABI require intensive guideline-directed medical therapy (GDMT) regardless of symptoms 5:

1. Statin therapy: Class I recommendation for all patients with any evidence of PAD, including borderline ABI 5, 6

   • High-intensity statin is preferred
   • If LDL-C remains ≥70 mg/dL on maximally tolerated statin, add ezetimibe or PCSK9 inhibitor 6

2. Antihypertensive therapy: Class I recommendation if hypertensive 5

   • ACE inhibitors or ARBs are particularly effective for reducing cardiovascular events in PAD patients 5

3. Smoking cessation: Absolutely critical if the patient smokes 5

   • Advise at every visit with structured cessation plan

4. Diabetes management: Optimize glycemic control if diabetic

Antiplatelet Therapy: The Uncertain Territory

This is where the evidence becomes nuanced. The 2016 guidelines explicitly state that for asymptomatic patients with borderline ABI (0.91–0.99), the usefulness of antiplatelet therapy to reduce MI, stroke, or vascular death is uncertain (Class IIb) 5.

However, the 2024 guidelines show evolution in thinking 6:

• For asymptomatic PAD (ABI ≤0.90), single antiplatelet therapy is now reasonable (Class IIa)
• The 2024 guidelines do not specifically address the borderline range separately, suggesting the uncertainty persists

Clinical decision-making for antiplatelet therapy in borderline ABI:

• If the patient has other cardiovascular disease (coronary disease, cerebrovascular disease): Antiplatelet therapy is already indicated for those conditions
• If the patient is truly asymptomatic with borderline ABI alone: The benefit is uncertain, but given the evidence of increased cardiovascular risk 7, 8, 9 and functional decline 7, aspirin 75–81 mg daily is reasonable as the risk-benefit ratio favors treatment in most patients without bleeding contraindications
• If exercise testing confirms PAD: Then antiplatelet therapy becomes clearly indicated

Emerging Evidence on Borderline ABI Significance

Recent research demonstrates that borderline ABI is not benign:

• Associated with 3-fold increased risk of mobility loss over 5 years 7
• Linked to endothelial dysfunction in 23% of patients 9
• Predicts major adverse limb events with HR 1.53 compared to normal ABI 10
• The risk follows an inverse J-shaped curve across the ABI spectrum, meaning even modest decrements matter 10

Monitoring and Follow-Up

Structured follow-up protocol:

1. Repeat ABI annually to detect progression 3
2. Assess for development of symptoms at each visit
3. Monitor cardiovascular risk factors aggressively
4. Foot examination if diabetic or if symptoms develop
5. Consider screening for abdominal aortic aneurysm if not previously done, as PAD is a risk factor 2

Common Pitfalls to Avoid

• Don't dismiss borderline ABI as "normal" – it carries significant cardiovascular and functional risk
• Don't skip exercise testing in symptomatic patients – resting ABI alone may miss significant disease
• Don't withhold statins – this is a Class I indication regardless of symptoms
• Don't assume antiplatelet therapy is clearly beneficial or clearly harmful – individualize based on overall cardiovascular risk and bleeding risk, but lean toward treatment in most cases
• Don't forget that borderline ABI may reflect early disease that will progress without aggressive risk factor modification

Special Populations

In patients with diabetes or chronic kidney disease: Be aware that medial arterial calcification may falsely elevate ABI. If clinical suspicion remains high despite borderline ABI, proceed to toe-brachial index or other testing 3, 11.

In women and Black individuals: The association between borderline ABI and adverse limb events appears consistent across sex and race, though women may have slightly higher relative risk 10.

5, 6, 1, 2, 1, 3, 7, 10