Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 75-year-old man with type 2 diabetes mellitus is evaluated in the emergency department for a draining chronic ulcer on the left foot, erythema, and fever. Drainage initially began 3 weeks ago. Current medications include metformin and glyburide.

On physical examination, he is not ill appearing. Temperature is 37.9 °C (100.2 °F); other vital signs are normal. The left foot is slightly warm and erythematous. A plantar ulcer that is draining purulent material is present over the fourth metatarsal joint. A metal probe makes contact with bone. The remainder of the examination is normal.

The leukocyte count is normal, and an erythrocyte sedimentation rate is 70 mm/h. A plain radiograph of the foot is normal.

Gram stain of the purulent drainage at the ulcer base shows numerous leukocytes, gram-positive cocci in clusters, and gram-negative rods.

Which of the following is the most appropriate management now?

A) Begin imipenem
B) Begin vancomycin and ceftazidime
C) Begin vancomycin and metronidazole
D) Perform bone biopsy

Answer and Critique

The correct answer is D) Perform bone biopsy. This item is available to MKSAP 15 subscribers as item 8 in the Infectious Disease section.

Contact with bone (when using a sterile, blunt, stainless steel probe) in the depth of an infected pedal ulcer in patients with diabetes mellitus is strongly correlated with the presence of underlying osteomyelitis, with a positive predictive value of 90%. Patients with diabetes require bone biopsy to obtain deep pathogens, identification of which is the only way to establish a definitive diagnosis and guide therapy. Although it may seem intuitive that drainage from a superficial site such as an ulcer or a sinus tract would contain the causative pathogens, superficial cultures usually do not include the deep organisms responsible for the infection. Failure to identify the causative deep-bone pathogens may lead to spread of infection to adjacent bones or soft tissues and the need for extensive debridement or amputation. The one exception is Staphylococcus aureus, which, even if found in superficial cultures, correlates well with findings on deep cultures.

This patient appears well enough to wait for the bone biopsy to be completed before starting empiric antibiotic therapy (and adjusting the antibiotics based on culture results) or until bone culture results become available. Empiric therapy should include activity against streptococci, methicillin-resistant S. aureus (MRSA), aerobic gram-negative bacilli, and anaerobes. Therapy with imipenem alone will not adequately cover MRSA, vancomycin and ceftazidime will not adequately cover anaerobic bacteria, and vancomycin and metronidazole will not adequately cover gram-negative organisms.

Key Point

• Cultures obtained from a sinus tract or ulcer base often do not reflect the bacterial etiology of an underlying osteomyelitis; bone biopsy is indicated to identify the causative pathogens and guide antibiotic therapy.

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