What is the most appropriate initial diagnostic test for a diabetic patient with a non‑healing right plantar foot ulcer, surrounding erythema, and loss of protective sensation?

Monofilament Testing is the Most Appropriate Diagnostic Test

For a diabetic patient presenting with a non-healing plantar ulcer, surrounding erythema, and reported loss of sensation, the 10-g monofilament test is the most appropriate diagnostic step to objectively confirm loss of protective sensation (LOPS), which is the single most common component cause of diabetic foot ulceration and fundamentally changes management. 1, 2

Why Monofilament Testing Takes Priority

Confirms the Underlying Pathophysiology

• Peripheral neuropathy with LOPS is present in 78% of diabetic patients with foot ulcers, making it the primary driver of ulceration rather than vascular disease alone 1, 2
• The patient's reported symptoms (numbness, inability to perceive touch) require objective confirmation through monofilament testing, as subjective history alone is insufficient for risk stratification and management planning 2
• The neurological examination in diabetic foot assessment is specifically designed to identify LOPS rather than early neuropathy, making monofilament testing the targeted diagnostic tool for this clinical scenario 2

Establishes Risk and Guides Management

• Confirming LOPS fundamentally changes the treatment approach, requiring specialized therapeutic footwear, more frequent monitoring intervals (not just annual visits), and mandatory referral to foot care specialists for ongoing preventive care and lifelong surveillance 1, 2
• The monofilament test is one of the two most sensitive tests for identifying feet at risk for ulceration and should be performed at every visit once LOPS is suspected 2
• The 10-g (5.07) monofilament threshold represents loss of approximately 98% of normal sensory ability, confirming that the patient has crossed into high-risk territory requiring aggressive intervention 3

Proper Testing Technique

• Perform monofilament testing at multiple plantar sites (hallux, first metatarsal head, fifth metatarsal head as minimum) with the patient's eyes closed 1
• Combine monofilament testing with at least one additional neurological assessment (pinprick, temperature, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis, as recommended by current ADA guidelines 1, 2
• Absent monofilament sensation at ≥2 applications at a single site confirms LOPS, while at least two normal tests with no abnormal test rules it out 2

Why the Other Options Are Secondary

ABI Testing—Important But Addresses a Different Question

• ABI is indicated for evaluating peripheral arterial disease (PAD), not for diagnosing the neuropathic cause of the ulcer 2
• While PAD assessment is essential (present in 20-40% of diabetic foot infections), the clinical presentation here—loss of sensation with inability to perceive touch—points directly to neuropathy as the primary diagnostic concern 1
• Initial PAD screening should include history for claudication and assessment of pedal pulses; ABI is obtained when these raise suspicion or in asymptomatic patients as secondary screening 1
• In this case, vascular assessment becomes critical after confirming LOPS, especially given the non-healing nature of the ulcer and surrounding erythema suggesting possible infection 1, 4

Pressure Point Assessment—Part of Treatment, Not Initial Diagnosis

• Assessment of pressure points becomes relevant after confirming LOPS through monofilament testing, as part of the comprehensive management strategy involving therapeutic footwear and offloading 2
• Pressure assessment helps identify areas requiring redistribution through specialized footwear but does not establish the underlying diagnosis of neuropathy 1
• The presence of erythema around the ulcer already indicates an area of increased pressure or tissue damage, but this is a consequence rather than the diagnostic finding 1

Critical Next Steps After Monofilament Testing

Immediate Actions

• Perform comprehensive foot examination including vascular assessment (pedal pulses, capillary refill, ABI if pulses diminished or absent) since the ulcer is non-healing 1, 4
• Evaluate for infection given the surrounding erythema—assess for warmth, tenderness, purulent drainage, and systemic signs; infection combined with ischemia requires intervention within 24 hours 1, 5
• Obtain plain X-rays of the foot to evaluate for osteomyelitis, especially given the chronic non-healing nature of the ulcer 1

Common Pitfalls to Avoid

• Do not assume neuropathy based solely on patient history—objective monofilament testing is essential for proper risk stratification and triggers specific management protocols 2
• Do not rely solely on palpable pulses to exclude PAD—up to 50% of diabetic foot ulcer patients have significant PAD despite seemingly adequate pulses on examination 5, 4
• Do not delay specialist referral once LOPS is confirmed—these patients require multidisciplinary care involving podiatry, vascular surgery if indicated, and wound care specialists 1

Ongoing Surveillance

• Patients with confirmed LOPS require visual foot inspection at every healthcare visit, not just annual screening 1
• The monofilament test should be repeated regularly as part of ongoing risk assessment 2