Monofilament Test is the Most Critical Diagnostic Test
The 10-g Semmes-Weinstein monofilament test is the most critical test to perform to diagnose this patient's condition, as it is specifically designed to identify loss of protective sensation (LOPS), which is the primary underlying pathophysiology in a diabetic patient presenting with a plantar foot ulcer and diminished sensation. 1
Clinical Reasoning
Why Monofilament Testing is Essential
The clinical presentation—a well-demarcated plantar ulcer with nearly diminished sensation and intact pulses—strongly suggests a neuropathic ulcer rather than an ischemic one. 1
Peripheral neuropathy with LOPS is the single most common component cause for diabetic foot ulceration, present in 78% of diabetic patients with ulcers. 1
The 10-g monofilament test is the most useful test to diagnose LOPS and is one of the two most sensitive tests for identifying feet at risk for ulceration. 1
The American Diabetes Association explicitly states that the neurological exam performed as part of the foot examination is designed to identify LOPS rather than early neuropathy, making it the primary diagnostic tool for this clinical scenario. 1
Proper Testing Technique
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. 1
Testing should be performed at three sites: the plantar aspects of the great toe, third metatarsal, and fifth metatarsal to maximize diagnostic accuracy. 2
Absent monofilament sensation confirms LOPS, while at least two normal tests (with no abnormal test) rules it out. 1
Why Other Options Are Less Critical
ABI (Ankle-Brachial Index) - Option A
ABI testing is indicated for peripheral arterial disease (PAD) evaluation, not for diagnosing the underlying cause of a neuropathic ulcer. 1
This patient has intact peripheral pulses, making significant PAD less likely as the primary etiology. 1
The American Diabetes Association recommends ABI testing only in patients with symptoms or signs of PAD (decreased walking speed, leg fatigue, claudication, absent pulses). 1
In this case, the well-demarcated plantar location and intact pulses point to neuropathic rather than ischemic pathophysiology. 1, 3
Assessment for Pressure Points - Option D
While pressure point assessment is important for management and prevention of ulcer recurrence, it does not establish the underlying diagnosis. 1
Pressure assessment becomes relevant after confirming LOPS through monofilament testing, as part of the treatment strategy involving therapeutic footwear and offloading. 1
Clinical Pitfalls to Avoid
False-positive responses can occur during monofilament testing and may affect results, so proper technique with sham applications should be used. 4
Do not assume neuropathy based solely on history—objective confirmation with monofilament testing is essential for risk stratification and management planning. 1, 5
Remember that the monofilament test has sensitivity ranging from 57% to 93% depending on methodology, so combining it with at least one additional neurological test improves diagnostic accuracy. 2
The presence of LOPS fundamentally changes management, requiring specialized footwear, more frequent monitoring (every 1-3 months if history of ulceration), and referral to foot care specialists. 1, 5