What is the most critical test to perform on a diabetic patient with a foot ulcer on the plantar surface and nearly diminished sensation?

Most Critical Test: 10-g Monofilament Test

In a diabetic patient presenting with a plantar foot ulcer and nearly diminished sensation, the monofilament test is the most critical test to perform, as it objectively confirms loss of protective sensation (LOPS)—the single most important diagnostic finding that fundamentally changes management and risk stratification. 1

Why Monofilament Testing Takes Priority

Diagnostic Necessity Despite Clinical Suspicion

• While the patient's history suggests neuropathy, objective confirmation with monofilament testing is essential for risk stratification and management planning—you cannot assume neuropathy based solely on history 1
• The 10-g monofilament test is explicitly designed to identify LOPS rather than early neuropathy, making it the primary diagnostic tool for this exact clinical scenario 2, 1
• 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 1

Impact on Management

• The presence of confirmed LOPS fundamentally changes management, requiring specialized therapeutic footwear, more frequent monitoring (every 1-3 months), and referral to foot care specialists 1, 3
• Patients with confirmed LOPS and active ulceration fall into the highest risk category (Category 3) in the International Working Group on the Diabetic Foot classification system 4, 3
• A multidisciplinary approach is mandated for individuals with foot ulcers and high-risk feet 2

Proper Testing Technique

Standard Protocol

• The monofilament test should ideally be performed with at least one other neurological assessment (pinprick, temperature, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis 2, 1, 4
• Absent monofilament sensation confirms LOPS, while at least two normal tests (with no abnormal test) rules it out 2, 1, 4

Critical Pitfall to Avoid

• The test includes a "sham" application that allows for false-positive answers, which can significantly affect results and diagnosis 5
• False-positive responses occur in a significant percentage of patients and should be accounted for in interpretation 5

Why Not the Other Options

ABI Testing (Option A)

• ABI testing is indicated for peripheral arterial disease (PAD) evaluation, not for diagnosing the underlying cause of a neuropathic ulcer 1
• The American Diabetes Association recommends ABI only for patients with symptoms of claudication or decreased/absent pedal pulses 2
• While vascular assessment is part of comprehensive foot examination, the clinical presentation (plantar ulcer with diminished sensation) points to neuropathic rather than ischemic etiology 2, 4

Pressure Point Assessment (Option C)

• Pressure assessment becomes relevant after confirming LOPS through monofilament testing, as part of the treatment strategy involving therapeutic footwear and offloading 1
• Evidence of increased plantar pressures (erythema, warmth, calluses) guides footwear selection but does not establish the primary diagnosis 2

Immediate Next Steps After Testing

Once LOPS is confirmed:

• Inspect for skin integrity, foot deformities, and signs of infection 2, 4
• Palpate pedal pulses to assess vascular status 2, 4
• Initiate appropriate wound care and offloading strategies 2
• Arrange referral to foot care specialist for ongoing preventive care and lifelong surveillance 2, 3