What is the most important diagnostic test for a diabetic patient with a plantar foot ulcer and nearly diminished sensation?

Monofilament Test (Answer: B)

The monofilament test is the most important diagnostic test in this clinical scenario because it objectively confirms loss of protective sensation (LOPS), which is the single most common cause of diabetic foot ulceration and fundamentally changes management. 1, 2

Why Monofilament Testing Takes Priority

The clinical presentation already strongly suggests LOPS based on "nearly diminished sensation," but objective confirmation is essential for proper risk stratification and management planning. 2 The 10-g monofilament test is explicitly designed to diagnose LOPS rather than early neuropathy, making it the primary diagnostic tool for this exact clinical scenario. 2

Key Supporting Evidence:

• Peripheral neuropathy with LOPS is present in 78% of diabetic patients with foot ulcers, making it the predominant component cause. 1, 2

• The monofilament test is one of the two most sensitive tests for identifying feet at risk for ulceration (along with clinical examination). 1

• Absent monofilament sensation confirms LOPS, while at least two normal tests rule it out. 2, 3

• The test should ideally be performed with at least one additional neurological assessment (pinprick, temperature perception with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis. 1, 3

Why Other Options Are Less Appropriate

ABI Testing (Option A):

• ABI is indicated for evaluating peripheral arterial disease (PAD), not for diagnosing the underlying cause of a neuropathic ulcer. 2
• The ulcer description (well-demarcated plantar location with central erythema and signs of healing) is classic for a neuropathic ulcer, not an ischemic ulcer. 1
• ABI should only be performed if the patient has symptoms or signs of PAD on initial screening (leg fatigue, claudication, absent pulses, etc.). 3, 4
• Important caveat: ABI results are often inaccurate in diabetic patients due to noncompressible vessels; toe systolic blood pressure is more reliable. 1

Assessment for Pressure Points (Option D):

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

CT Scan (Option C):

• Not indicated in this clinical presentation unless there is concern for osteomyelitis or deep tissue infection, which is not suggested by the description of a healing ulcer with central erythema.

Clinical Impact of Confirming LOPS

The presence of LOPS fundamentally changes management, requiring: 2

• Specialized therapeutic footwear
• More frequent monitoring (every 1-3 months for high-risk patients) 4
• Referral to foot care specialists 4
• Patient education on daily foot inspection and proper foot care 4

Common Pitfall to Avoid

Do not assume neuropathy based solely on history or clinical impression—objective confirmation with monofilament testing is essential for proper risk stratification and management planning. 2 The prognostic value of the monofilament test is well-established, with relative risks for future ulceration ranging from 2.5 to 7.9 in patients with positive results. 5