When should a wound culture be obtained?

When to Obtain a Wound Culture

Wound cultures should only be obtained from clinically infected wounds prior to starting antibiotic therapy, while avoiding cultures of uninfected wounds. 1

Indications for Wound Cultures

Recommended:

• Obtain cultures from clinically infected wounds showing:
  • Purulent drainage
  • Erythema, warmth, swelling, pain
  • Systemic symptoms (fever, worsening mental status)
  • Signs of sepsis 1, 2
• Collect cultures before starting empiric antibiotic therapy whenever possible 1
• Obtain cultures when there is risk of antibiotic-resistant organisms:
  • Chronic infections
  • Recent antibiotic exposure
  • Prior history of MRSA
  • High local prevalence of resistant organisms 1

Not Recommended:

• Culturing clinically uninfected wounds (unless for specific epidemiological purposes) 1
• Routine cultures of all wounds regardless of infection status 2
• Cultures for mild infections in antibiotic-naïve patients at low risk for MRSA (these infections are typically caused by staphylococci and streptococci) 1

Proper Technique for Wound Culture Collection

Do:

• Cleanse and debride the wound before obtaining specimens 1, 2
• Obtain deep tissue specimens using one of these methods:
  • Tissue biopsy (gold standard) 1, 3, 4
  • Curettage (scraping tissue from ulcer base with sterile scalpel/curette) 1
  • Needle aspiration for purulent collections 2
• Collect at least 0.5 mL of fluid or 0.5 g of tissue for adequate sampling 2
• Use sterile containers or appropriate transport media 1, 2
• Send for both aerobic and anaerobic cultures 1, 2

Don't:

• Swab undebrided wounds or surface drainage 1, 2
• Collect specimens after starting antibiotics (reduces culture yield) 2
• Delay antimicrobial therapy to obtain cultures if the patient appears septic 2
• Rely on aerobic cultures alone (will miss potentially important anaerobic pathogens) 2

Special Considerations

Diabetic Foot Infections

For diabetic foot infections, the Infectious Diseases Society of America provides specific guidance:

• Cultures are essential for moderate to severe infections and when multidrug-resistant organisms are suspected 1
• Deep tissue specimens after cleansing and debridement provide the most accurate results 1

Skin and Soft Tissue Infections

• Blood cultures have low yield (7.0%) even in admitted patients 5
• Even in febrile patients or injection drug users, blood culture yields remain low (3.5% and 8.7% respectively) 5
• Wound cultures from purulent infections have higher yield (29.6%) 5

Chronic Wounds

• Consider culturing when additional signs beyond classic infection are present:
  • Increasing pain
  • Presence of necrotic tissue
  • Delayed or deteriorating healing
  • Changes in wound bed appearance 4
• The Levine technique (rotating a swab with pressure in a 1 cm² area) is more reliable than the Z-technique when tissue biopsy is not feasible 4

Interpretation and Clinical Application

• Use culture results to guide definitive antibiotic therapy, considering both the organisms identified and their antimicrobial susceptibilities 1, 2
• Remember that surface cultures may reflect colonization rather than true infection 2
• Consider that molecular microbiological techniques can detect more organisms and provide results faster than standard cultures 1, 2

By following these evidence-based guidelines for wound cultures, clinicians can optimize antimicrobial therapy while avoiding unnecessary testing and inappropriate antibiotic use.