MD Notes

creation date: 2026-05-14 16:52
tags: Pathologies

Diabetic Foot Infection

Background

Definitions

Diabetic foot infection is a common cause of hospitalization and lower extremity amputations for patients with diabetes.

This encapsulates both and in combination, skin and soft tissue infection and osteomyelitis of the foot. Diabetic foot infection affects type 1 diabetes and type 2 diabetes equally.

Risk Factors, Etiology, and Pathogenesis

A number of complications of diabetes predispose infection.

  • Sensory neuropathy reduces perception of pain and temperature - patients take longer to recognize injury or infection.
  • Peripheral neuropathy changes the shape of the foot - creates new pressure points
  • Peripheral artery disease impairs blood flow - impairs healing of ulcers and infections
  • Hyperglycemia impairs neutrophil function and immune response

Ulcers that are deep, chronic, recurrent, or due to trauma are especially prone to infection. Other points of entry for microorganisms are dry, cracked skin due to diminished sweat secretion from autonomic neuropathy.

These infections can spread contiguously.

Microbiology

Organisms depend on type, severity, and recent antibiotic exposure. The common organisms are as follows.

Superficial infections

  • Staphylococcus aureus
  • Streptococcus agalactiae
  • Streptococcus pyogenes

Deep or chronically infected ulcers
Above organisms and:

  • Enterococci
  • Enterobacterales
  • Pseudomonas aeurginosa
  • Anaerobes

Extensive local inflammation, necrosis, malodourous drainage, or gangrene
Above and anaerobes such as:

  • Anaerobic streptococci
  • Bacteroides spp.
  • Clostridium spp.

Concerning organisms not covered by some empiric therapy are:

  • MRSA
  • Pseudomonas aeruginosa
  • Anaerobic organisms

Clinical Presentation

Signs & Symptoms

Infections can present on a spectrum, ranging from a localized superficial skin infection to deeper infection involving the fascia, bone, joints, and bloodstream.

Skin and soft tissue infection
Cardinal manifestations of inflammation include:

  • Erythema
  • Warmth
  • Swelling
  • Tenderness
  • Pus in ulcer

Signs of infection may appear blunted due to peripheral vascular disease and neuropathy.

Osteomyelitis
Osteomyelitis is discussed in more depth separately. However, underlying osteomyelitis in the context of diabetic foot infection are unclear and may present with just:

  • Soft tissue infection
  • Chronic ulcer
  • “Sausage toe” (erythema and nonpitting edema that obliterates normal contour of the digit)

History & Physical Exam

Encounter should focus on:

  1. Determining extent and severity of infection
  2. Assessing the microbial etiology
  3. Identifying risk factors that result in infection

History:

  • Recent trauma or new footwear
  • Duration of current lesion(s)
  • Associated systemic symptoms (fever, chills)
  • Prior treatment/antibiotics use
  • Neuropathy/peripheral vascular disease symptoms

Physical:

  • Skin examination
  • Inspection for visible bone; also, “probe-to-bone” test - use of a sterile blunt metal probe to see if a hard, gritty surface (bone) can be felt
  • Neurologic evaluation
  • Vascular evaluation

Diagnosis

Criteria

Diagnosis of infection is made clinically. Note that the presence of an ulcer is not necessary.

IWGFD/IDSA classification system for presence and severity of infection
1 - uninfected

  • No systemic or local signs/symptoms of infection

2 - infected, mild

  • At least two of the following:
    • Local swelling or induration
    • Erythema 0.5-2 cm around the wound
    • Local tenderness or pain
    • Local increased warmth
    • Purulent discharge
  • No other cause of inflammatory response (eg. trauma, gout)

3 - infected, moderate

  • Infection and one or both of the following:
    • Erythema extending ≥2 cm from the wound margin
    • Tissue deeper than skin and subcutaneous tissues involved
  • 3(O) if includes osteomyelitis

4 - infected, severe

  • Foot infection with associated systemic manifestation, as manifested by ≥2 of the following:
    • Temperature >38C or <36C
    • Heart rate >90 bpm
    • Respiratory rate >20 breaths/min, or PaCO2 < 32 mmHg
    • WBC >12,000 /mm3, or <4 G/L, or >10% immature forms
  • 4(O) if includes osteomyelitis

Work-up

Wound culture
Wound cultures are only performed on select patient for moderate and severe infections or when there are concerns for multi-drug resistant organisms to guide antibiotic therapy.

The preferred method for collection is aspiration of abscess or curettage from an ulcer base. Superficial swabs are not appropriate as they are unreliable in predicting pathogen responsible for infection.

Osteomyelitis
Discussed more in depth separately. Tests include:

  • Visible bone/probe-to-bone test
  • Inflammatory markers (ESR/CRP)
  • Imaging (plain radiographs in suspected osteomyelitis, MRI for evaluation)
  • Bone biopsy

Differential

Noninfectious conditions that cause inflammatory changes in lower extremity skin include:

  • Diabetic neuroarthropathy/Charcot arthropathy (eval: no ulcer, fever, leukocytosis; may require joint aspiration)
  • Venous stasis (eval: usually calves, no signs of infection)
  • Deep vein thrombosis (eval: calves, ultrasound)
  • Gout (eval: joint aspiration)
  • Fracture/other trauma (eval: x-ray)

Red Flags / Complications

Complications include:

  • Worsening of infection (eg. progression to osteomyelitis)
  • Bone fracture
  • Sepsis
  • Gangrene
  • Necrosis

Diabetic foot ulcers are a major cause of amputation.

Management

Treatment of Skin and Soft Tissue Infection Without Osteomyelitis

Prevention

The mainstay of preventing infection consist of preventing foot wounds. This consist of:

  • Regular foot examinations (both self and by clinician)
  • Proper foot fitting
  • Attentive wound management if ulcer develops
  • Vascular evaluation and intervention for significant peripheral artery disease
  • Optimal glycemic control

Antibiotic Therapy

Considerations exist for additional pathogen coverage:

  • Anaerobic bacteria - for moderate or severe infection with necrosis, gangrene, malodourous drainage, limb ischemia, or presence of gas on radiographic imaging
  • MRSA - known MRSA colonization or prior infection, individual risk factors
  • Pseudomonas - severe infection or particular risk (eg. moist appearance, prolonged exposure to warm tropical)
Mild Infection

No MRSA coverage:

  • Dicloxacillin 500 mg PO q6h
  • Cephalexin 500 mg PO q6h
  • Cefadroxil 500 mg PO BID

MRSA coverage:

  • TMP-SMX 1-2 double strength tab PO BID
  • Amoxicillin (875-1000 mg PO BID) plus doxycycline (100 mg PO BID)
Moderate Infection

No additional coverage:

  • Amoxicillin-clavulanate 875 mg PO q12h

MRSA:

  • Amoxi-clav 875 mg PO q12h plus doxycycline 100 mg PO q12h
  • Amoxi-clav 875 mg PO q12h plus TMP-SMX 1-2 double strength tab PO BID

Pseudomonas:

  • Amoxi-clav 875 mg PO q12h plus ciprofloxacin 750 mg PO q12h

MRSA and pseudomonas:

  • Levofloxacin 750 mg PO daily plus doxycycline 100 mg PO q12h
  • Levofloxacin 750 mg PO daily plus TMP-SMX 1-2 double strength tab PO BID
  • Add metronidazole 500 mg PO q8-12h for anaerobic coverage
Severe Infection

Severe infection require broad spectrum IV antibiotic therapy:

  • Vancomycin (dosing) plus piperacillin-tazobactam 4.5 g q6h
  • Vancomycin plus cefepime 2g q8h plus metronidazole 500 mg q8-12h

Surgery

Surgical intervention is primarily to remove infected nonviable tissue. All patients with severe and most with moderate foot infection should be evaluated by a surgeon.

Surgical debridement is indicated urgently for:

  • Deep or extensive abscess
  • Extensive bone or joint involvement
  • Crepitus, necrosis, or gangrene
  • Necrotizing fasciitis
  • Severe lower extremity ischemia
  • Compartment syndrome

Surgery may also allow for soft tissue cultures.

In cases of severe disease in which antibiotics and limited debridement is insufficient (eg. osteomyelitis with extensive bone destruction, extensive gangrene, life-threatening sepsis, severe lower limb ischemia), amputation may be necessary.

Osteomyelitis

Surgery

Most patients with overlying osteomyelitis require surgery. Generally, debridement of bone occurs with debridement of soft tissue infection.

In cases where soft tissue debridement is not necessary, the benefit of operation must be weighed against risk of poor wound healing following surgery.

Guidelines suggest surgery may be foregone if all of the following criteria is met:

  • Infection confined to forefoot (metatarsals and/or phalanges)
  • No indication for soft tissue debridement
  • No significant peripheral artery disease
  • No exposed bone

Antibiotics

IV antibiotics should be administered initially. Oral therapy may be considered post-operation.

Without pseudomonas coverage:

  • Ceftriaxone 2 g IV once daily plus metronidazole 500 mg PO/IV q8-12h
  • Add vancomycin IV for MRSA coverage

With pseudomonas coverage:

  • Cefepime 2g IV q8h plus metronidazole 500 mg PO/IV q8-12h
  • Piperacillin-tazobactam 4.5 g IV q6h

References

Tools / Guidelines

Additional Reading

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