Diabetic foot infections (DFIs) are an increasingly common problem, but proper care can save limbs and lives as suggested by updated guidelines published earlier this year by the Infectious Diseases Society of America.1 Diabetic foot infections typically begin in a wound, most often a neuropathic ulceration. Most require some surgical intervention, ranging from minor debridement to major amputation. As recommended by the guideline, establishing a multidisciplinary team improves outcomes.
Summarized below are several of the recommendations in this update (specific recommendations are graded as to the strength of recommendation and the level of evidence with “strong” being the highest. The author’s comments are shown below [under the headings “Comment”]):
I. In which diabetic patients with a foot wound should I suspect infection and how should I classify it?
- Evidence of infection generally includes classical signs of inflammation (redness, warmth, swelling, tenderness, or pain) or purulent secretions but may also include additional or secondary signs (eg, nonpurulent secretions, friable or discolored granulation tissue, undermining of wound edges, foul odor) (strong, low).
The panel suggests that caregivers should be aware of factors that increase the risk for DFI and especially consider infection when they are present; these include the following: a wound for which the probe to bone test is positive; an ulceration present for more than 30 days; a history of recurrent foot ulcers; a traumatic foot wound; the presence of peripheral vascular disease in the affected limb in a previous lower extremity amputation; loss of protective sensation; the presence of renal insufficiency; or a history of walking barefoot. In addition, the panel recommends routine use of a validated classification system. The IDSA classification is included in Table 1:
When and how should I obtain specimen(s) for culture from a patient with a diabetic foot wound?
- Send appropriately obtained specimens for culture before starting empirical antibiotic therapy, if possible. Cultures may be unnecessary for a mild infection in a patient who has not recently received antibiotic therapy (strong, low).
- Send a specimen for culture that is from deep tissue, obtained by biopsy or curettage, and after the wound has been cleansed and debrided. We suggest avoiding swab specimens, especially of inadequately debrided wounds, provide less accurate results (strong, moderate).
Antimicrobials should be based on appropriate culture results. While all wounds are colonized, it is important to use microbiological methods that will more likely identify true pathogens. As such, swab cultures are discouraged in favor of biopsy specimens. The guideline panel also states that recent studies have demonstrated that standard cultures identify only a small percentage of microorganisms present in wounds; future use of molecular tests may more rapidly and accurately identify pathogens and even virulence factors.
VI. How should I initially select, and when should I modify, an antibiotic regimen for a diabetic foot infection?
- We recommend that clinicians select an empirical antibiotic regimen based on the severity of the infection and the likely etiologic agent(s) (strong, low).
- a. For mild to moderate infections in patients who have not recently received antibiotic treatment, we suggest that therapy just targeting aerobic gram-positive cocci is sufficient (weak, low).
- b. For most severe infections, we recommend starting broad-spectrum empiric antibiotic therapy, pending culture results and antibiotic susceptibility data (strong, low).
- We recommend that definitive therapy be based on both the results of an appropriately obtained culture and sensitivity testing of a wound specimen and the patient’s clinical response to the empirical regimen (strong, low).
- We suggest basing the route of therapy largely on infection severity. We prefer parenteral therapy for all severe, and some moderate, DFIs, at least initially (weak, low), with a switch to oral agents when the patient is systemically well and culture results are available. Physicians can probably use highly bioavailable oral antibiotics alone in most mild infections (strong, moderate).
- We suggest continuing antibiotic therapy until, but not beyond, resolution of findings of infection, but not through complete healing of the wound (weak, low). We suggest an initial antibiotic course for a soft tissue infection of approximately 1 to 2 weeks for mild infections and 2 to 3 weeks for moderate to severe infections (weak, low).
The guideline appropriately recommends prescribing antibiotic therapy for all infected wounds, but cautions that this is often insufficient if not combined with appropriate wound care.
Suggested empiric antimicrobial regimens are based on clinical severity; a brief summary of suggested antimicrobials are as follows (not inclusive):
- Mild (oral agents): cephalexin, amoxicillin/clavulanate, clindamycin (usually active for community-acquired MRSA); for MRSA: trimethoprim/sulfamethoxazole, doxycycline
- Moderate: ampicillin/sulbactam, ertapenem, imipentem-cilastatin, vancomcyin for MRSA (also listed linezolid and daptomycin), piperacillin/tazobactam for Pseudomonas
- Severe: “very broad-spectrum coverage” to include coverage of MRSA, gram-negative bacilli including Pseudomonas, and obligate anaerobes
Empiric therapy directed at Pseudomonas aeruginosa is usually unnecessary except for patients with risk factors for true infection with this organism (eg, patients who have been soaking their feet, have severe infection, and have failed nonpseudomonal therapy). The guideline also recommends providing empirical therapy directed against MRSA in a patient with a history of MRSA infection; when the local prevalence of MRSA colonization or infection is high; or if the infection is clinically severe (weak, low). Personally, I feel the risk of MRSA is “high” in most cases and usually provide therapy directed against MRSA pending appropriate culture results.
The most appropriate duration of therapy for DFI is not well defined, but the guideline does provide some general recommendations:
- For mild infections, 1 to 2 weeks; moderate infections, 1 to 3 weeks; and severe infections, 2 to 4 weeks; for bone infection, if the bone has been completely resected, 2 to 5 days; if bone has residual infection, 4 to 6 weeks; but in no surgery and the presence of residual dead bone, 3 months or longer.
VII. When should I consider imaging studies to evaluate a diabetic foot infection, and which should I select?
- We recommend that all patients presenting with a new DFI have plain radiographs of the affected foot to look for bony abnormalities (deformity, destruction) as well as for soft 3 tissue gas and radio-opaque foreign bodies (strong, moderate).
- We recommend using magnetic resonance imaging (MRI) as the study of choice for patients who require further (ie, more sensitive or specific) imaging, particularly when soft tissue abscess is suspected or the diagnosis of osteomyelitis remains uncertain (strong, moderate).
- When MRI is unavailable or contraindicated, physicians might consider the combination of a radionuclide bone scan and a labeled white blood cell scan as the best alternative.
Plain radiographs have only moderately helpful performance characteristics. Magnetic resonance imaging provides the greatest accuracy for detection of bone involvement and has the advantage of optimal detection of soft tissue infection (eg, abscess).
How should I diagnose and treat osteomyelitis of the foot?
- For a diagnostic imaging test for diabetic foot osteomyelitis (DFO), we recommend using MRI (strong, moderate). However, MRI is not always necessary for diagnosing or managing DFO (strong, low).
- We suggest that the most definitive way to diagnose DFO is by the combined findings on bone culture and histology (strong, moderate). When bone is debrided to treat osteomyelitis, we suggest sending a sample for culture and histology (strong, low).
- For patients not undergoing bone debridement, we suggest that physicians consider obtaining a diagnostic bone biopsy when faced with specific circumstances, for example, diagnostic uncertainty, inadequate culture information, and failure of response to empiric treatment (weak, low).
- Physicians can consider using either primarily surgical or primarily medical strategies for treating DFO in properly selected patients (weak, moderate). When a radical resection leaves no remaining infected tissue, we suggest prescribing antibiotic therapy for only a short duration (2–5 days) (weak, low). When there is persistent infected or necrotic bone, we suggest prolonged (≥4 weeks) antibiotic treatment (weak, low).
Bone resection has traditionally been considered a requirement for cure of DFO; however, the guideline lists 4 situations in which nonsurgical management might be considered:
- There is no acceptable surgical target (ie, radical cure would cause unacceptable loss of function)
- Presence of limb ischemia due to unreconstructable vascular disease and desire to avoid amputation
- Infection confined to the forefoot, and there is minimal soft tissue loss
- Surgery carries excessive risk.
1. Lipsky BA, Berendt AR, Cornia PB, et al.. 2012 Infectious Diseases Society of America clinical practice guideline for the diagnosis and treatment of diabetic foot infections. Clin Infect Dis
. 2012; 54: 132–173.