How to tell if a diabetic foot wound is infected

Diagnosing Infection in Diabetic Foot Wounds

A diabetic foot wound should be diagnosed as infected when there is purulent drainage or at least 2 of the classic signs of inflammation: redness, warmth, swelling/induration, and pain/tenderness. 1

Primary Clinical Signs of Infection

Infection in diabetic foot wounds should be determined based on clinical assessment rather than laboratory findings. Look for:

Cardinal Signs of Infection

• Purulent secretions (pus) - definitive sign of infection on its own
• Inflammatory signs (at least 2 needed to diagnose infection):
  • Erythema (redness)
  • Warmth
  • Swelling or induration
  • Pain or tenderness

Secondary Signs of Infection

In patients with neuropathy or ischemia who may not display classic inflammatory signs, look for:

• Friable or discolored granulation tissue
• Undermining of wound edges
• Foul odor
• Non-purulent secretions
• Delayed healing despite proper care 1

Assessment Algorithm

1. Inspect the wound:

   • Look for purulent drainage (diagnostic on its own)
   • Assess for ≥2 cardinal signs of inflammation
   • Check for secondary signs if classic signs are absent

2. Evaluate wound characteristics:

   • Depth and extent of tissue involvement
   • Presence of necrotic tissue
   • Undermining of edges
   • Probe the wound (especially to detect bone involvement)

3. Assess for systemic signs of infection:

   • Fever
   • Chills
   • Elevated white blood cell count
   • Metabolic instability (severe hyperglycemia, acidosis)

4. Determine infection severity (guides management):

   • Mild: Local infection with minimal tissue involvement
   • Moderate: Deeper or more extensive infection
   • Severe: Systemic inflammatory response or metabolic instability 1

Obtaining Cultures

Cultures are essential for guiding antibiotic therapy in infected wounds:

• When to culture:

  • All infected wounds except perhaps mild, previously untreated infections
  • Do NOT culture clinically uninfected wounds 1

• Proper technique:

  • Cleanse and debride the wound first
  • Obtain tissue specimens from the debrided base by:
    • Curettage (scraping with sterile curette/scalpel)
    • Biopsy (bedside or operative)
    • Aspiration for purulent collections
  • Avoid swabbing undebrided ulcers or wound drainage
  • If swabbing is the only option, use swabs designed for aerobic and anaerobic organisms 1

Common Pitfalls to Avoid

1. Misdiagnosing colonization as infection:

   • All wounds are colonized with bacteria
   • Presence of bacteria alone does not indicate infection
   • Antibiotics are unnecessary for uninfected wounds 1, 2

2. Relying solely on laboratory values:

   • Diagnosis of infection is primarily clinical
   • Many diabetic patients with serious foot infections lack systemic signs 1

3. Improper culture technique:

   • Surface swabs often reflect colonizing organisms, not pathogens
   • Failure to debride before obtaining cultures leads to misleading results 1, 3

4. Missing deep infection:

   • Deep tissue infection may be present despite minimal surface signs
   • Consider imaging (plain X-rays initially, MRI for suspected osteomyelitis) 1

5. Overlooking vascular insufficiency:

   • Always assess arterial supply to the affected foot
   • Ischemia can mask inflammatory signs and impair healing 1

By following this systematic approach to diagnosing diabetic foot infections, you can identify infections early, determine their severity, and initiate appropriate treatment to reduce the risk of serious complications including amputation.