The Monofilament Test is the Most Critical Test
In a diabetic patient presenting with a non-healing foot ulcer, numbness, and inability to perceive touch and sensation, the monofilament test is the most critical diagnostic test to perform because it objectively confirms loss of protective sensation (LOPS), which is the single most common component cause of diabetic foot ulceration (present in 78% of cases) and fundamentally changes management. 1, 2
Why the Monofilament Test Takes Priority
The clinical presentation already strongly suggests LOPS based on the patient's symptoms (numbness, inability to perceive touch), but objective confirmation with the 10-g Semmes-Weinstein monofilament test is essential for risk stratification and management planning—you cannot assume neuropathy based solely on history. 2
Key Diagnostic Points:
The monofilament test is one of the two most sensitive tests for identifying feet at risk for ulceration and is explicitly designed to diagnose LOPS rather than early neuropathy. 1, 2
The American Diabetes Association recommends performing the monofilament test with at least one additional neurological assessment (pinprick, temperature, vibration with 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis. 1, 2
Absent monofilament sensation confirms LOPS, while at least two normal tests (with no abnormal tests) rules it out. 2
The monofilament test has superior sensitivity (100%) compared to other modalities in identifying patients with foot ulcers, making it the optimal screening tool even though it has slightly lower specificity (77.7%). 3
Why Not the Other Options?
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 IWGDF guidelines recommend ABI measurement in diabetic patients with foot ulcers to evaluate for PAD (with <0.9 considered abnormal), but this addresses vascular compromise rather than the neuropathic component. 1
In this case, there is no mention of absent pulses, claudication, or other PAD symptoms—the presentation points to a neuropathic ulcer (numbness, loss of sensation, central erythema). 1
ABI becomes relevant if the ulcer fails to heal despite optimal management or if there are signs of ischemia, but it is not the primary diagnostic test for this presentation. 1
Assessment of Pressure Points (Option C):
Pressure assessment becomes relevant after confirming LOPS through monofilament testing, as part of the treatment strategy involving therapeutic footwear and offloading. 2
While foot deformities and prominent metatarsal heads increase plantar pressures and ulceration risk, identifying these pressure points does not establish the diagnosis of LOPS. 1
The presence of LOPS must be confirmed first, as this fundamentally changes management to include specialized footwear, more frequent monitoring, and referral to foot care specialists. 2
Clinical Algorithm for This Patient:
Perform the 10-g monofilament test immediately at multiple sites on the foot (at least 3-4 points, though 8-10 points may increase sensitivity). 1, 4
Combine with at least one additional neurological test (vibration with 128-Hz tuning fork has the highest likelihood ratio of 16-35 for detecting large-fiber peripheral neuropathy). 1, 5
Once LOPS is confirmed, assess for PAD by palpating dorsalis pedis and posterior tibial pulses, and consider ABI if pulses are diminished or absent. 1
Evaluate pressure points and foot deformities to guide offloading strategies and therapeutic footwear prescription. 1, 2
Refer to a multidisciplinary foot care team for ongoing management, as patients with LOPS and active ulceration are high-risk. 1, 2
Critical Pitfall to Avoid:
Do not skip objective monofilament testing simply because the patient reports numbness. The subjective complaint must be confirmed with standardized testing to properly risk-stratify the patient and trigger the appropriate intensive management protocols that prevent progression to amputation. 2