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, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis. 1, 2, 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 described shows "signs of healing" with "central erythema," suggesting adequate perfusion rather than ischemic disease. 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
• Common pitfall: ABI values are often inaccurate in diabetic patients due to noncompressible vessels; toe systolic blood pressure is more reliable when vascular assessment is needed. 1

CT Scan (Option C):

• Not indicated for routine diabetic foot ulcer evaluation unless there is concern for deep infection, osteomyelitis, or Charcot arthropathy—none of which are suggested by this presentation. 1

Pressure Point Assessment (Option D):

• While important, pressure assessment becomes relevant after confirming LOPS through monofilament testing, as part of the treatment strategy involving therapeutic footwear and offloading. 2
• Assessment for foot deformities (bunions, hammertoes, prominent metatarsals) is part of the comprehensive foot examination but does not take diagnostic priority over confirming LOPS. 1, 3

Clinical Impact of Confirming LOPS

The presence of LOPS fundamentally changes management, requiring:

• Specialized therapeutic footwear 2
• More frequent monitoring (every 1-3 months for high-risk patients) 3, 4
• Referral to foot care specialists 4
• Patient education on daily foot inspection and proper foot care 4

The monofilament test has proven prognostic value, with relative risk for foot ulceration ranging from 2.5 to 7.9 in patients with positive results, and relative risk for lower extremity amputation ranging from 1.7 to 15.1. 5