Monofilament Test (Answer: B)
The monofilament test is the most critical test to perform in this clinical scenario, as it objectively confirms loss of protective sensation (LOPS), which is the primary diagnostic finding needed to establish the diagnosis of diabetic neuropathic foot ulcer and guide subsequent management. 1, 2
Why Monofilament Testing is the Answer
The clinical presentation already strongly suggests LOPS based on history ("nearly diminished sensation" and "unable to perceive touch"), but objective confirmation with the 10-g Semmes-Weinstein monofilament test is essential for formal diagnosis and risk stratification rather than relying on subjective history alone. 2
Key Diagnostic Principles
The monofilament test is explicitly designed to diagnose LOPS rather than early neuropathy, making it the primary diagnostic tool for this exact clinical scenario of an established plantar ulcer with suspected sensory loss. 1, 2
LOPS is present in 78% of diabetic patients with foot ulcers, and the monofilament test is one of the two most sensitive tests for identifying feet at risk for ulceration. 2, 3
Absent monofilament sensation definitively confirms LOPS, while normal results (at least two normal tests with no abnormal tests) rule it out. 1, 2
Proper Testing Technique
The monofilament should be applied perpendicular to the skin surface with sufficient force to cause buckling, held for approximately 2 seconds, and tested at three sites on each foot. 1
Apply the filament along the perimeter of the ulcer, not on the ulcer itself, callus, scar, or necrotic tissue. 1
Ideally combine with at least one additional neurological assessment (pinprick, temperature, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis. 1, 2
Why the Other Options Are Incorrect
ABI (Ankle-Brachial Index) - Option A
ABI testing is indicated for evaluating peripheral arterial disease (PAD), not for diagnosing the underlying cause of a neuropathic ulcer. 2
The clinical presentation describes a well-demarcated plantar ulcer with central erythema and signs of healing, which is the classic appearance of a neuropathic ulcer from pressure and trauma, not an ischemic ulcer. 4
ABI should only be performed if the patient has symptoms of claudication, decreased walking speed, leg fatigue, or absent/decreased pedal pulses on initial screening. 1, 5
Pressure Assessment - Option D
While pressure assessment becomes relevant after confirming LOPS through monofilament testing, it is part of the treatment strategy (therapeutic footwear, offloading) rather than the diagnostic workup. 2
The presence of LOPS must be objectively documented first, as this fundamentally changes management to require specialized footwear, more frequent monitoring, and referral to foot care specialists. 2
Clinical Impact of Confirming LOPS
Patients with documented LOPS require immediate risk stratification and specialized preventive care, including referral to foot care specialists for ongoing preventive care and lifelong surveillance. 2, 5
The diagnosis triggers specific management interventions: specialized therapeutic footwear with demonstrated pressure relief, more frequent monitoring intervals (every 1-3 months rather than annually), and multidisciplinary team involvement. 1, 5
The inability to feel a 5.07 monofilament represents a sensory threshold more than 50 times greater than normal, meaning roughly 98% of sensory ability has been lost, underscoring the critical nature of objective documentation. 6