Diabetic Foot Infections Workup

The patient’s complete blood count (CBC) and erythrocyte sedimentation rate (ESR) vary according to the type of diabetic foot infection.^[6] Gram stain and cultures can aid in determining the etiology of infection in skin and soft-tissue infections, while in acute osteomyelitis and cellulitis, blood cultures can help to identify causative organisms. In chronic osteomyelitis, bone biopsy can be used to find the infecting microbe.

Imaging studies do not play a role in the diagnosis of cellulitis, but they are a valuable tool in the assessment of the other infection types.

The complete blood count (CBC) count and erythrocyte sedimentation rate (ESR) are slightly or moderately elevated in cellulitis. However, the elevations are not diagnostic and, therefore, are unhelpful. Blood culture results are usually negative; if positive, they usually indicate the presence of group A or group B streptococci. Cultures of skin via aspiration or biopsy are generally unrewarding.

Imaging studies are not applicable in cellulitis.

The CBC and ESR are mildly or moderately elevated in these tissue infections. If bullae are present, Gram stain and culture results from aspirated exudate from a bullous lesion may provide clues to the etiology of the infection. Blood culture results may be positive.

In a patient with diabetes considered to have a deep soft-tissue infection, plain radiography, computed tomography (CT) scanning, or magnetic resonance imaging (MRI) may be performed to rule out a compartment syndrome, which may present as extreme pain and tenderness of the affected limb, and to demonstrate the presence of gas or a foreign body in the deep tissues.^[7] A finding of excessive gas signifies a mixed aerobic-anaerobic infection, in contrast to gas gangrene (clostridial myonecrosis).

Aspiration of a sample from the leading edge of the erythematous border in a patient with cellulitis is usually not necessary, but a sample may be aspirated if the likely organism must be identified on initial presentation. However, the yield is low, likely to be less than 5%.

Samples from deep-skin and soft-tissue infections may be aspirated. Gram stains and/or cultures may be used to identify the organism.

The CBC usually reveals leukocytosis, and the ESR is moderately or highly elevated, in acute osteomyelitis.^[6] Blood culture results are usually negative; when positive, the findings most frequently indicate the presence of S aureus.

For affected long bones, plain radiographic findings generally become abnormal after 10-14 days. Soft-tissue swelling and periosteal elevation are the earliest signs of acute osteomyelitis on a plain radiograph. Bone scans are preferred to gallium or indium scans in the assessment of acute osteomyelitis, as the latter imaging studies offer no additional information, and the findings are not more specific than those of bone scans. In addition, indium scans often show false-negative results in acute or chronic osteomyelitis. Bone-scan findings are positive within 24 hours.

Bone biopsy is not necessary in acute osteomyelitis, because the pathogens are predictable.

In chronic osteomyelitis, the CBC is often within the reference range. Usually, the ESR is very highly elevated and may exceed 100 mm/h.^[6] The platelet count is also often elevated in chronic osteomyelitis. Blood culture results are usually negative.

When osteomyelitis becomes chronic, plain radiographic findings are invariably abnormal, as shown in the image below.

Bone scans are usually unnecessary unless diagnostic confusion exists with another disorder, as when a bone tumor must be differentiated from chronic osteomyelitis before definitive bone biopsy. An MRI would also be helpful in such a situation.

Bone biopsy performed under aseptic conditions in the operating room is the preferred way to identify the causative pathogen in chronic osteomyelitis. P aeruginosa is usually not the causative organism; however, B fragilis is an important bone pathogen in chronic osteomyelitis in patients with diabetes, as are E coli, P mirabilis, and K pneumoniae.

Because surgical debridement is critical in treating chronic osteomyelitis, bone biopsy specimens are usually not obtained during the surgical debridement procedure.

Subperiosteal elevation and/or infection may involve the cortex in acute osteomyelitis. Involucrum and/or sequestrum may be present in the cortical bone in cases of chronic osteomyelitis.