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Difficulty: HardDiagnostic Workup and Test Sequencing

A 61-year-old man is evaluated in the outpatient clinic for a persistent ulcer on the plantar aspect of his left first metatarsophalangeal joint, which he first noticed six weeks ago. His medical history is significant for type 2 diabetes mellitus complicated by peripheral neuropathy, as well as stage 3 chronic kidney disease. He reports no fevers, chills, or systemic symptoms. Vital signs are within normal limits. Physical examination reveals a 2 cm×2 cm2 \text{ cm} \times 2 \text{ cm} ulcer with raised borders and a granulating, mildly exudative base. Surrounding erythema extends 1 cm1 \text{ cm} from the ulcer margin. Sensation to a 10-gram monofilament is absent in the bilateral distal lower extremities. During the examination, a sterile, blunt stainless-steel probe is introduced into the ulcer and advances seamlessly until a hard, gritty surface is encountered. Laboratory studies show a leukocyte count of 8,500/mm38,500/\text{mm}^3 and an erythrocyte sedimentation rate of 82 mm/h82 \text{ mm/h}. A plain radiograph of the left foot reveals soft tissue swelling but no radiopaque foreign bodies, subcutaneous gas, or distinct cortical destruction. Which of the following is the most appropriate next step in the diagnostic workup?

  1. Percutaneous or surgical bone biopsyAnswer
  2. B
    Magnetic resonance imaging (MRI) of the left foot
  3. C
    Superficial swab culture of the ulcer exudate
  4. D
    Technetium-99m labeled leukocyte scintigraphy
  5. E
    Measurement of serum procalcitonin concentration

Answer

The most appropriate next step is percutaneous or surgical bone biopsy.
The correct response is percutaneous or surgical bone biopsy. In a patient with a diabetic foot ulcer, a positive probe-to-bone test (palpating a hard, gritty surface) combined with an elevated erythrocyte sedimentation rate is highly specific for underlying osteomyelitis. Because the diagnosis is clinically apparent, advanced imaging is generally not required unless surgical debridement mapping is needed. Instead, the priority of the diagnostic workup shifts to identifying the causative organism to guide long-term, targeted antimicrobial therapy. Bone biopsy (percutaneous or surgical) is the gold standard for microbiologic diagnosis and should be obtained prior to initiating empiric antibiotics in hemodynamically stable patients.

Step-by-Step Solution

1
Analyze the clinical presentation and physical examination findings.
The patient has a chronic diabetic foot ulcer with a positive probe-to-bone test and an elevated erythrocyte sedimentation rate.
A positive probe-to-bone test in the setting of a diabetic foot ulcer has a high positive predictive value for osteomyelitis, establishing the diagnosis clinically.
2
Evaluate the results of the initial imaging.
A plain radiograph was performed, which is appropriately the first-line imaging modality to rule out gas or gross deformities, but showed no distinct cortical destruction.
Plain films have low sensitivity for acute/subacute osteomyelitis, so negative results do not rule out the condition when clinical suspicion is very high.
3
Determine the next necessary diagnostic test based on guidelines.
Because the diagnosis of osteomyelitis is clinically confirmed, the next priority is obtaining reliable microbiologic data via bone biopsy before starting long-term targeted antibiotics.
Advanced imaging like MRI is skipped in cases of unambiguous clinical osteomyelitis to prevent unnecessary costs and delays in obtaining definitive deep tissue cultures.

Key Concept

In patients with a diabetic foot ulcer and a positive probe-to-bone test, the diagnosis of osteomyelitis is clinically established. The next best step in diagnostic workup is a bone biopsy for culture to guide targeted antibiotic therapy, rather than obtaining an MRI.
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Diagnostic Workup and Test Sequencing

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