What is the most critical test for a diabetic patient with a well-demarcated plantar foot ulcer, diminished sensation, and intact peripheral pulses?

The Monofilament Test is the Most Critical Diagnostic Test

The 10-g monofilament test is the most critical test to perform in this patient, as it definitively diagnoses loss of protective sensation (LOPS)—the single most common cause of diabetic foot ulceration, present in 78% of cases. 1, 2

Why the Monofilament Test Takes Priority

The clinical presentation strongly suggests a neuropathic ulcer:

• Well-demarcated plantar location (classic for pressure-related neuropathic ulcers) 1
• Nearly diminished sensation (suggests LOPS but requires objective confirmation) 2
• Intact peripheral pulses (effectively rules out peripheral arterial disease as the primary etiology) 1

While the history suggests neuropathy, objective confirmation with monofilament testing is essential for risk stratification and management planning—you cannot assume neuropathy based solely on history. 2

The Diagnostic Algorithm

Step 1: Confirm LOPS with monofilament testing

• The 10-g Semmes-Weinstein monofilament is one of the two most sensitive tests for identifying feet at risk for ulceration 1, 2
• Test at plantar sites on both feet 3
• Combine with at least one additional neurological test (pinprick, temperature perception, vibration with 128-Hz tuning fork, or ankle reflexes) 1, 2
• Absent monofilament sensation confirms LOPS 2, 4

Step 2: The diagnosis fundamentally changes management

• Presence of LOPS requires specialized footwear, more frequent monitoring, and referral to foot care specialists 2
• Pressure assessment (option D) becomes relevant after confirming LOPS, as part of treatment strategy involving therapeutic footwear and offloading 2

Why the Other Options Are Incorrect

ABI (Option A) is not indicated here:

• ABI testing evaluates for peripheral arterial disease, not neuropathy 1
• This patient has intact peripheral pulses, making significant PAD unlikely 1
• Guidelines recommend ABI only when patients have symptoms/signs of PAD or when evaluating a foot ulcer for healing potential 1
• The IWGDF explicitly states that patients with foot ulcers should undergo ABI measurement, but this is to assess vascular adequacy for healing, not to diagnose the underlying cause of a neuropathic ulcer 1

CT scan (Option C) has no role in initial diagnosis:

• CT is not part of standard diabetic foot ulcer evaluation 1
• May be considered later for osteomyelitis assessment if infection is suspected, but not for initial diagnosis 1

Pressure point assessment (Option D) is secondary:

• While foot deformities and pressure points are important risk factors, they are assessed after confirming LOPS 1, 2
• Assessment for bunions, hammertoes, and prominent metatarsals is part of the comprehensive foot examination but does not establish the diagnosis 1, 3

Critical Clinical Pearls

• Monofilament testing has superior reproducibility (kappa = 0.59) compared to other bedside tests like pinprick, vibration, or position sense (kappa = 0.28-0.36) 5
• The test can be simplified to 4 sites per foot (8 total) while still detecting 90% of patients with abnormal sensation 5
• Peripheral neuropathy is a component cause in 78% of diabetic foot ulcers, and the triad of peripheral sensory neuropathy, minor trauma, and foot deformity is present in >63% of cases 1
• The neurological exam is specifically designed to identify LOPS rather than early neuropathy, making it the primary diagnostic tool for this exact clinical scenario 2