Should I suspect contamination if a wound culture grows 8 different organisms?

Multiple Organisms in Wound Culture: When to Suspect Contamination

A wound culture growing 8 different organisms strongly suggests contamination rather than true infection, especially if the specimen was collected using a swab rather than tissue biopsy. 1

Understanding Wound Culture Results

Factors Suggesting Contamination

• Cultures from superficial lesions often yield a mixture of pathogens, colonizing organisms, and contaminants 1
• Swab specimens are particularly prone to contamination due to their collection of surface flora rather than deeper tissue 1
• The presence of multiple organisms (especially 8 different ones) is more consistent with contamination than true infection 1
• Qualitative culture of wound swabs has limited value due to inevitable contamination of any open wound 1

Proper Specimen Collection

• Tissue specimens obtained by biopsy or curettage are more reliable than swabs for determining true pathogens 1
• Specimens should be collected only after the wound has been properly cleansed and debrided 1
• Aseptically obtained deep tissue specimens usually contain only true pathogens 1
• Swabs should be avoided as they have high risk of contamination and limited specimen capacity 1

Clinical Assessment of Wound Infection

Signs of True Infection

• Presence of purulence or spreading cellulitis are key indicators of infection 1
• Systemic signs of infection (fever, elevated white blood cell count) suggest true infection rather than contamination 1
• Discharge of purulent fluid is diagnostic of surgical site infection 1
• Spreading inflammation beyond what is seen in normal healing suggests infection 1

Wound Categories and Infection Risk

• Clean wounds have only 1.5% infection rate and rarely involve multiple organisms 1
• Contaminated wounds have 15.2% infection rate and often involve multiple organisms 1
• Dirty wounds have 40% infection rate and are frequently polymicrobial 1, 2

Recommendations for Management

When to Repeat Cultures

• If the initial culture shows 8 organisms, consider repeating with proper technique (tissue biopsy rather than swab) 1
• Do not obtain repeat cultures unless the patient is not clinically responding to treatment 1
• For a more accurate assessment, cleanse the wound thoroughly before obtaining the specimen 1

Treatment Considerations

• Base treatment decisions on clinical signs of infection rather than solely on culture results showing multiple organisms 1, 2
• For contaminated or dirty wounds with clinical signs of infection, consider anaerobic coverage even if anaerobes are not identified by routine microbiology 1, 2
• In wounds with multiple organisms, focus treatment on the most virulent pathogens (e.g., S. aureus, β-hemolytic streptococci) rather than treating all cultured organisms 1

Common Pitfalls

• Relying solely on swab cultures for diagnosis of wound infection 1
• Failing to properly cleanse and debride the wound before specimen collection 1
• Treating colonization (presence of bacteria without clinical infection) with antibiotics 1, 3
• Not considering the wound type and location when interpreting culture results 1, 2

Remember that all wounds are colonized with microorganisms, and the mere presence of bacteria (even multiple species) does not necessarily indicate infection requiring antibiotic treatment 1, 3. Clinical assessment remains the cornerstone of diagnosis.