Monofilament Test is the Most Critical Diagnostic Test
In a diabetic patient presenting with a non-healing foot ulcer, numbness, and inability to perceive touch and sensation, the 10-g monofilament test (Option B) is the most critical test to establish the diagnosis of loss of protective sensation (LOPS), which is the underlying cause of the ulcer.
Why Monofilament Testing is the Answer
The clinical scenario describes a patient who already has:
- A non-healing ulcer (the complication has occurred)
- Numbness and inability to perceive sensation (symptoms of neuropathy)
- No mention of vascular symptoms or absent pulses
The diagnostic priority is to objectively confirm LOPS, which is present in 78% of diabetic patients with foot ulcers and is the single most common component cause of diabetic foot ulceration 1. The American Diabetes Association explicitly states that the 10-g monofilament test is "the most useful test to determine LOPS" and is "one of the two most sensitive tests in identifying the foot at risk for ulceration" 1.
Why the Other Options Are Incorrect
ABI (Ankle-Brachial Index) - Option A
- ABI is indicated for diagnosing peripheral arterial disease (PAD), not for confirming neuropathy 1
- The American Diabetes Association recommends ABI only when patients have "signs and symptoms of PAD" such as claudication, rest pain, or absent pulses 1
- This patient has no vascular symptoms mentioned, making ABI a secondary consideration 1
- ABI does not diagnose the neuropathic cause of this ulcer 2
Assessment of Pressure Points - Option D
- Pressure assessment becomes relevant after confirming LOPS through monofilament testing, as part of treatment planning for therapeutic footwear and offloading 2
- This is a management tool, not a diagnostic test for the underlying pathology 3
The Diagnostic Algorithm
Step 1: Confirm LOPS with monofilament testing
- Perform 10-g monofilament test at multiple sites (minimum 3-4 sites per foot) 1, 4, 5
- Combine with at least one other neurological test (pinprick, temperature, vibration with 128-Hz tuning fork, or ankle reflexes) 1
- Absent monofilament sensation confirms LOPS 1, 2
Step 2: Assess vascular status
- Palpate dorsalis pedis and posterior tibial pulses 1
- If pulses are diminished/absent or patient has claudication symptoms, then proceed to ABI with toe pressures 1
Step 3: Document structural deformities and pressure points
- This guides therapeutic footwear prescription and offloading strategies 3
Critical Clinical Pitfall
Do not assume neuropathy based solely on patient-reported numbness—objective confirmation with monofilament testing is essential for proper risk stratification and management planning 2. The presence of confirmed LOPS fundamentally changes management, requiring specialized therapeutic footwear, referral to foot care specialists, and monitoring every 1-3 months rather than annually 1, 3.
The monofilament test is inexpensive, portable, painless, easy to administer, and provides the best predictive ability for ulceration risk 6. This objective documentation of LOPS is what drives all subsequent management decisions, including the intensity of follow-up, footwear prescription, and specialist referral 2, 7, 3.