Probe-to-Bone Test
The most critical test to perform is the probe-to-bone test, which should be done immediately at the bedside by inserting a sterile metal probe into the ulcer base to assess for underlying osteomyelitis. 1
Why Probe-to-Bone is the Priority
The probe-to-bone test has high sensitivity and specificity for detecting osteomyelitis in diabetic foot ulcers, and a positive result mandates further investigation with imaging. 1
In diabetic patients with plantar foot ulcers and diminished sensation, the primary diagnostic concern is ruling out osteomyelitis, which occurs in approximately 20% of infected diabetic foot ulcers and fundamentally changes management. 1
The presence of central erythema with signs of non-healing raises concern for infection masquerading as healing tissue—erythema extending beyond the wound margin (>2 cm) indicates infection rather than healing. 1
Why the Other Options Are Not the Answer
ABI (Option A) is Unreliable in This Context
ABI is unreliable in diabetic patients with neuropathy due to medial arterial calcification (Mönckeberg sclerosis), which produces falsely normal or elevated readings despite underlying stenotic disease. 2, 1
While vascular assessment is important, the patient has intact peripheral pulses, making critical ischemia less likely as the immediate cause of non-healing. 3
If vascular assessment were needed, toe pressure or transcutaneous oxygen pressure (TcPO₂) would be more reliable than ABI in diabetic patients with neuropathy. 1
Monofilament Test (Option B) is Already Done
The clinical presentation already confirms diminished sensation, so repeating sensory testing with a monofilament does not advance the diagnosis. 1
The monofilament test confirms neuropathy but does not explain why this specific ulcer is non-healing with central erythema. 1
Assessment for Pressure Points (Option D) is Secondary
While offloading and pressure assessment are critical for management, they do not establish the diagnosis of why the ulcer is non-healing with central erythema. 3
Pressure point assessment should follow after ruling out infection and osteomyelitis. 3
Immediate Next Steps After Probe-to-Bone
Obtain weight-bearing plain radiographs in at least two views immediately to screen for osteomyelitis, soft-tissue gas, foreign bodies, and bone-architecture abnormalities including Charcot foot deformities. 3, 1
Plain radiographs detect osteomyelitis in 50-70% of cases and provide essential baseline information about bone involvement. 3
Collect deep tissue specimens after surgical debridement (not superficial swabs) to ensure accurate culture results if infection is suspected. 3
Common Pitfall to Avoid
Never assume adequate perfusion based solely on palpable pulses in diabetic patients—up to 50% of diabetic foot ulcers have coexisting peripheral artery disease, and even experienced clinicians can detect pulses despite significant ischemia. 3, 1
However, in this specific case with intact pulses and a non-healing ulcer with central erythema, infection and osteomyelitis are more likely culprits than critical ischemia, making probe-to-bone the most critical immediate test. 1