What are the general guidelines for interpreting limb perfusion studies (e.g., skin‑perfusion pressure, transcutaneous oxygen tension, laser‑Doppler flow, near‑infrared spectroscopy, plethysmography) in patients being evaluated for peripheral arterial disease, diabetic foot ulcers, wound healing, or post‑revascularization, considering age, diabetes mellitus, smoking status, hypertension, hyperlipidemia, and prior limb interventions?

Interpreting Limb Perfusion Studies in Peripheral Arterial Disease

Critical Threshold Values for Clinical Decision-Making

The most clinically actionable perfusion thresholds follow a sigmoid curve: healing is likely when toe pressure ≥30 mmHg, TcPO₂ ≥25 mmHg, or skin perfusion pressure ≥40 mmHg, while healing is severely impaired below these values. 1

Wound Healing Prediction Thresholds

Favorable healing parameters:

• Toe pressure >55 mmHg – ulcers will often heal 1
• TcPO₂ >50 mmHg – ulcers will often heal 1
• Skin perfusion pressure ≥40 mmHg – increases pre-test probability of healing by ≥25% 1

Severe ischemia requiring urgent intervention:

• Toe pressure <30 mmHg – healing severely impaired, consider urgent vascular imaging and revascularization 1
• TcPO₂ <25 mmHg – healing severely impaired, consider urgent vascular imaging and revascularization 1
• TcPO₂ <30 mmHg – healing usually severely impaired 1

Amputation Risk Prediction Thresholds

High-risk parameters requiring urgent intervention:

• Ankle pressure <50 mmHg – consider urgent vascular imaging and revascularization 1
• ABI <0.5 – consider urgent vascular imaging and revascularization 1
• Combined ankle pressure <50 mmHg OR ABI <0.5 – increases pre-test probability of major amputation by ~40% 1

Moderate ischemia thresholds:

• ABI <0.6 – indicates significant ischemia with respect to wound healing potential 1
• ABI >0.6 – has less predictive value; measure toe pressure and/or TcPO₂ in these patients 1

Post-Revascularization Target Values

After revascularization, achieving minimum perfusion values increases the likelihood of healing: 1

• Skin perfusion pressure ≥40 mmHg 1
• Toe pressure ≥30 mmHg 1
• TcPO₂ ≥25 mmHg 1

Critical Interpretation Principles

No Single Test is Definitive

No perfusion measure alone should determine revascularization or amputation decisions. 1 Given the variability of PAD in distribution, severity, and symptoms, no single measure performs with consistent accuracy for predicting healing or amputation 1. The decision to perform major amputation before attempting revascularization should never be made on perfusion measures alone 1.

The 6-Week Rule for Borderline Perfusion

In patients without ischemic symptoms, with palpable foot pulses, or with perfusion measurements suggesting only mild PAD (ABI >0.6 with toe pressure >55 mmHg or TcPO₂ >50 mmHg), evaluate the effect of maximal 6-week optimal wound care. 1 If wound healing response is poor, reassess perfusion and strongly consider duplex ultrasound or angiography 1.

Integration with Clinical Context

Consider vascular imaging and revascularization in all patients with foot ulcers and PAD, irrespective of bedside test results, when the ulcer does not improve within 6 weeks despite optimal management. 1

Special Population Considerations

Diabetes and Arterial Calcification

When ABI >1.40 (non-compressible vessels due to medial arterial calcification, common in diabetes and chronic kidney disease), obtain toe-brachial index (TBI) instead. 1 A TBI <0.70 strongly suggests PAD in a foot acclimatized in warm surroundings 1. This is critical because falsely elevated ABI values can mask severe ischemia in diabetic patients 1.

Diabetic Microangiopathy Misconception

Diabetic microangiopathy should not be assumed to be the cause of poor wound healing in patients with foot ulcers. 1 Always evaluate for macrovascular PAD, as it is the primary driver of non-healing wounds and is amenable to revascularization 1.

Algorithmic Approach to Perfusion Study Interpretation

Step 1: Initial Screening

• Measure ABI in all patients with diabetic foot ulcers 1
• If ABI <0.9, PAD is confirmed 1
• If ABI >1.40, proceed to TBI measurement 1

Step 2: Severity Stratification

• If ABI <0.6: significant ischemia present, proceed to anatomic imaging 1
• If ABI 0.6-0.9: measure toe pressure and/or TcPO₂ 1
• If ankle pressure <50 mmHg OR ABI <0.5: urgent vascular imaging indicated 1

Step 3: Wound Healing Potential Assessment

• If toe pressure <30 mmHg OR TcPO₂ <25 mmHg: consider urgent vascular imaging and revascularization 1
• If toe pressure 30-55 mmHg OR TcPO₂ 25-50 mmHg: intermediate risk, close monitoring with 6-week trial of optimal wound care 1
• If toe pressure >55 mmHg AND TcPO₂ >50 mmHg: favorable healing potential, continue conservative management 1

Step 4: Revascularization Decision

• Never base revascularization decisions on perfusion measures alone 1
• Obtain anatomic imaging (duplex ultrasound, CTA, MRA, or DSA) to assess stenosis location, severity, and distribution 1
• Evaluate entire lower extremity arterial circulation with detailed visualization of below-knee and pedal arteries 1

Common Pitfalls to Avoid

Relying on ABI alone in diabetic patients: Medial arterial calcification produces falsely elevated values; always obtain TBI when ABI >1.40 1.

Assuming normal perfusion studies exclude PAD: Clinical judgment and 6-week wound healing response should guide further evaluation 1.

Making amputation decisions based solely on perfusion values: Multiple factors including infection, tissue necrosis volume, and functional status must be considered 1.

Ignoring the sigmoid curve nature of perfusion thresholds: Values between 30-55 mmHg (toe pressure) or 25-50 mmHg (TcPO₂) represent intermediate risk requiring individualized assessment 1.

Failing to reassess perfusion after 6 weeks of optimal wound care: Poor healing response mandates repeat perfusion testing and consideration of anatomic imaging 1.