In a diabetic patient with a non‑healing plantar foot ulcer and loss of protective sensation, what is the most critical test to perform to diagnose peripheral neuropathy?

The Monofilament Test is the Most Critical Test to Perform

In a diabetic patient presenting with a non-healing plantar foot ulcer and numbness, the monofilament test (Option B) is the most critical diagnostic test because it objectively confirms loss of protective sensation (LOPS), which is the single most common component cause of diabetic foot ulceration and fundamentally changes management.

Why the Monofilament Test Takes Priority

Primary Diagnostic Purpose

• The 10-g monofilament test specifically identifies LOPS, which is present in 78% of diabetic patients with foot ulcers and represents the most critical risk factor for ulceration 1, 2.
• While the patient reports numbness (subjective symptom), the monofilament test provides objective confirmation that is essential for risk stratification and management planning 2.
• The American Diabetes Association explicitly states that the neurological examination is designed to identify LOPS rather than early neuropathy, making it the primary diagnostic tool for this exact clinical scenario 2.

Clinical Examination Technique

• The monofilament should be applied to at least 3 plantar sites: the great toe, third metatarsal, and fifth metatarsal areas 3.
• Combine monofilament testing with at least one other neurological assessment (pinprick, temperature perception, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis 1, 2.
• Absent monofilament sensation confirms LOPS, while at least two normal tests with no abnormal tests rules it out 2.

Why the Other Options Are Less Critical at This Stage

ABI Testing (Option A) - Important But Secondary

• ABI is indicated for evaluating peripheral artery disease (PAD), not for diagnosing the underlying cause of a neuropathic ulcer 2.
• The IWGDF guidelines recommend evaluating for PAD in patients with foot ulcers by measuring ankle systolic pressure and ABI 1.
• However, the clinical presentation (plantar location, numbness, inability to perceive touch) strongly suggests a neuropathic rather than ischemic ulcer 1.
• ABI should be performed as part of comprehensive evaluation, but confirming LOPS takes diagnostic priority because it determines the fundamental pathophysiology and treatment approach 1.

Assessment of Pressure Points (Option C) - Part of Treatment, Not Diagnosis

• Pressure assessment becomes relevant after confirming LOPS through monofilament testing, as part of the treatment strategy involving therapeutic footwear and offloading 2.
• The IWGDF guidelines recommend prescribing therapeutic footwear that demonstrates plantar pressure-relieving effects to prevent ulcer recurrence 1.
• This is a management intervention, not a diagnostic test for the underlying neuropathy 1.

Critical Management Implications of Confirming LOPS

Immediate Risk Stratification

• Patients with LOPS require classification into IWGDF risk categories (risk 1-3 based on additional factors like PAD, deformity, or prior ulceration) 1.
• This patient would be classified as IWGDF risk 3 (LOPS with current ulcer), requiring screening every 1-3 months 1.

Treatment Changes Based on LOPS Diagnosis

• Specialized therapeutic footwear with demonstrated plantar pressure reduction is required 1.
• Patient education must include: never walking barefoot or in thin-soled slippers, daily foot inspection, and immediate reporting of any pre-ulcerative lesions 1.
• Referral to foot care specialists for ongoing preventive care and lifelong surveillance 2.

Common Pitfalls to Avoid

• Do not assume neuropathy based solely on patient-reported numbness—objective confirmation with monofilament testing is essential 2.
• Do not skip the monofilament test in favor of only vascular assessment when the clinical presentation suggests neuropathic ulceration 1.
• Do not use monofilament testing alone—combine with at least one other neurological test (vibration, pinprick, or temperature) to improve diagnostic accuracy 1, 2.
• Remember that the heel has the least sensitivity in normal individuals (1/6th that of toes), so ensure testing includes multiple plantar sites 4.

Diagnostic Algorithm for This Patient

1. Perform 10-g monofilament testing at 3-4 plantar sites (great toe, third and fifth metatarsal heads) 3.
2. Add at least one confirmatory test (128-Hz tuning fork for vibration, pinprick, or temperature sensation) 1, 2.
3. Palpate pedal pulses (dorsalis pedis and posterior tibial) to screen for PAD 1.
4. If pulses are diminished or absent, proceed to ABI and consider toe pressures (more accurate than ABI in diabetes due to non-compressible vessels) 1.
5. Assess for foot deformities (bunions, hammertoes, prominent metatarsals) that increase plantar pressures 1.

The monofilament test objectively confirms the diagnosis of LOPS, which is the fundamental pathophysiologic mechanism in this patient's non-healing plantar ulcer and determines all subsequent management decisions 1, 2.