What is the purpose and process of taking a tissue culture and sensitivity (CS) test to guide antibiotic treatment for a patient with a suspected infection?

Technique of Taking Tissue Culture and Sensitivity

Obtain deep tissue specimens using aseptic technique rather than superficial swabs, as tissue samples are both more sensitive and specific for identifying true pathogens and their antibiotic susceptibilities. 1

Specimen Collection Method

Preferred Technique: Deep Tissue Sampling

• Collect at least 5 separate deep tissue specimens intraoperatively using separate sterile instruments for each sample, targeting areas around the infection site and adjacent to any implants 1
• Obtain specimens through curettage or tissue biopsy from the base of the wound, avoiding superficial swabs which yield mixed results of pathogens, colonizers, and contaminants 1
• Stop antimicrobial therapy at least 2 weeks before sampling to avoid false-negative culture results, unless the patient is clinically unstable 1

When Swabs Are Used (Less Optimal)

• If tissue sampling is not feasible, swabs should be taken from deep in the wound after debridement, not from the surface 1
• Swab cultures have low sensitivity and high contamination risk compared to tissue specimens 1

Timing and Clinical Context

Before Starting Antibiotics

• Collect specimens before initiating empirical antimicrobial therapy whenever clinically justifiable to maximize culture sensitivity 1
• In clinically unstable patients with signs of sepsis, administer antibiotics immediately after sampling rather than delaying treatment 1

For Non-Responding Infections

• If infection fails to respond after 48-72 hours of treatment, consider discontinuing antibiotics temporarily (if patient is stable), then obtain optimal culture specimens after an antibiotic-free interval 2, 3
• Repeat cultures are only indicated when patients are not responding to treatment or for infection control surveillance of resistant pathogens 1

Sample Processing and Transport

Immediate Handling

• Send specimens to the microbiology laboratory promptly in sterile transport containers 1
• Inoculate tissue specimens directly into blood culture bottles for greatest accuracy 1
• For enrichment of slow-growing pathogens (e.g., Cutibacterium acnes), use enrichment broth cultures that can be sub-cultured up to 14 days 1

Critical Clinical Information to Include

• Provide the laboratory with: site and type of infection, specimen type obtained, whether patient is taking antibiotics, and location of the wound 1
• This information influences specimen processing and reporting protocols 1

Laboratory Analysis

Immediate Assessment (Within Hours)

• Request Gram stain for rapid preliminary results - this simple procedure has 93% positive predictive value for bacterial growth and can guide initial therapy while awaiting cultures 1
• Gram stain can identify whether Gram-positive cocci or Gram-negative rods are present with 75-82% accuracy 1

Culture Methods

• Use standard and selective growth media with antimicrobial sensitivity testing 1
• Homogenization is the preferred method for processing tissue samples, with techniques including vortexing, sonication, or beadmill processing to disrupt biofilms 1
• Culture thresholds: endotracheal aspirates ≥10^6 cfu/ml, bronchoalveolar lavage ≥10^4-10^5 cfu/ml 1

Histopathology (When Indicated)

• For suspected osteomyelitis, obtain bone specimens for both culture and histopathology 1
• Presence of >5 polymorphonuclear neutrophils per high-power field confirms infection with 100% specificity 1

Common Pitfalls to Avoid

Sampling Errors

• Never culture sinus tracts - contamination with commensal skin flora is common and results do not reflect deep tissue pathogens 1
• Avoid superficial wound swabs which miss facultative and anaerobic organisms 1
• Do not rely on single specimens - the multifocal nature of infections requires multiple samples to avoid sampling errors 1

Timing Mistakes

• Collecting cultures after prolonged antibiotic therapy (>72 hours) significantly reduces sensitivity 1
• Delaying specimen collection in unstable patients to wait for antibiotic-free intervals increases mortality risk 1

Processing Errors

• Samples contaminated with high percentages of squamous epithelial cells should be interpreted with caution 1
• Failure to provide clinical context to the laboratory leads to suboptimal processing protocols 1

Special Considerations

For Suspected Osteomyelitis

• Aseptically obtained bone specimens are required for definitive diagnosis 1
• Avoid using soft tissue or sinus tract specimens to guide antibiotic therapy for bone infections - they do not accurately reflect bone culture results 1

For Implant-Related Infections

• Sonication of removed implants may be useful as an adjunct to tissue culture, especially in antibiotic-pretreated patients, though tissue cultures remain more sensitive 1
• Sample tissue from sites adjacent to implants using separate instruments 1

For Polymicrobial Infections

• Multiple tissue specimens increase detection of all causative organisms in polymicrobial infections 1
• Anaerobic organisms require specific transport media and processing - alert the laboratory if anaerobes are suspected (foul-smelling, necrotic wounds) 1