What is the most critical test to perform in a diabetic patient with a well-demarcated plantar foot ulcer, nearly diminished sensation, and inability to perceive touch?

The Monofilament Test is the Most Critical Diagnostic Test

The monofilament test (Option B) is the most critical test to perform in this patient, as it is specifically designed to diagnose loss of protective sensation (LOPS), which is the underlying pathophysiology responsible for this plantar ulcer. 1, 2

Why the Monofilament Test is Essential

The clinical presentation—a well-demarcated plantar ulcer with nearly diminished sensation and inability to perceive touch—strongly indicates a neuropathic ulcer caused by LOPS. The monofilament test directly confirms this diagnosis:

• The 10-g Semmes-Weinstein monofilament is the single most useful test to diagnose LOPS, which is present in 78% of diabetic patients who develop foot ulcers 2, 3
• The neurological exam in diabetic foot evaluation is specifically designed to identify LOPS rather than early neuropathy, making it the primary diagnostic tool for this exact clinical scenario 1, 2
• Absent monofilament sensation confirms LOPS, while normal results rule it out 1, 3

Proper Testing Technique

The monofilament test should be performed systematically:

• Combine the 10-g monofilament with at least one additional neurological assessment (pinprick, temperature, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis 1, 2, 3
• Test three sites on each foot, applying the filament perpendicular to the skin with sufficient force to cause buckling 1
• Apply along the perimeter of the ulcer, not on it, to avoid contamination and obtain accurate readings 1
• Protective sensation is present if the patient correctly answers two out of three applications at each site 1

Why Other Options Are Incorrect

ABI Testing (Option A)

• ABI is indicated for peripheral arterial disease (PAD) evaluation, not for diagnosing the underlying cause of a neuropathic ulcer 2
• The American Diabetes Association recommends ABI only when patients have symptoms of claudication or decreased/absent pedal pulses 1
• This patient's presentation (plantar location, well-demarcated borders, central erythema with healing) is classic for neuropathic rather than ischemic ulceration 4

Pressure Assessment (Option D)

• Pressure assessment becomes relevant AFTER confirming LOPS through monofilament testing, as part of the treatment strategy involving therapeutic footwear and offloading 2
• It is a management tool, not a diagnostic test for the underlying pathophysiology 2

Critical Clinical Implications

Confirming LOPS fundamentally changes management:

• Patients with LOPS require specialized therapeutic footwear, more frequent monitoring (every 1-3 months rather than annually), and referral to foot care specialists 1, 2, 3
• Do not assume neuropathy based solely on history—objective confirmation with monofilament testing is essential for proper risk stratification and management planning 2
• The inability to feel a 5.07 monofilament represents loss of more than 98% of normal sensory ability, indicating severe neuropathy 5

Common Pitfall to Avoid

Never skip objective sensory testing even when the history strongly suggests neuropathy. The monofilament test provides quantifiable, reproducible documentation of LOPS severity that guides both immediate management and long-term surveillance strategies 1, 2, 3