Is Staphylococcus aureus (S. aureus) in a thoracentesis sample considered a pathogen or contaminant?

Staphylococcus aureus in Thoracentesis Samples: Pathogen or Contaminant?

Staphylococcus aureus isolated from a thoracentesis sample should generally be considered a true pathogen rather than a contaminant, especially when found in significant quantities and in the appropriate clinical context.

Evaluation Framework for S. aureus in Pleural Fluid

When S. aureus is isolated from a thoracentesis sample, several factors should be considered to determine its clinical significance:

1. Specimen Quality and Collection Method

• Thoracentesis samples are generally considered sterile specimens when properly collected
• Unlike respiratory specimens such as sputum, pleural fluid is not normally colonized with bacteria
• Proper collection technique minimizes the risk of contamination 1

2. Growth Characteristics

• Heavy growth of S. aureus strongly suggests true infection
• Quantitative cultures showing significant bacterial load support pathogen status
• S. aureus as the predominant organism on direct Gram stain increases likelihood of true infection 1

3. Supporting Clinical and Laboratory Evidence

• Presence of purulent fluid (empyema) strongly suggests infection
• Pleural fluid parameters supporting infection:
  • pH <7.28
  • Glucose <40 mg/dL
  • LDH >1000 IU/L
  • Presence of polymorphonuclear leukocytes 1
• Systemic signs of infection (fever, elevated WBC, elevated inflammatory markers)

S. aureus as a Significant Pathogen

S. aureus is explicitly recognized as a potential pathogen in pleural infections by multiple guidelines:

• The Infectious Diseases Society of America (IDSA) guidelines specifically list S. aureus among the important pathogens that can cause respiratory infections, including pleural space infections 1
• Unlike organisms such as enterococci, viridans streptococci, coagulase-negative staphylococci, and Candida species (which are often contaminants), S. aureus is classified as a potential true pathogen 1

Differentiating from Contamination

Contamination should be suspected when:

• Very light growth of S. aureus occurs
• Multiple other skin flora are present in the sample
• Clinical picture is inconsistent with infection
• Pleural fluid parameters are normal (normal pH, glucose, protein, LDH)

Clinical Implications and Management

If S. aureus is isolated from thoracentesis fluid:

1. Initiate appropriate antimicrobial therapy targeting S. aureus
2. Consider drainage of the pleural space, especially if parameters suggest empyema
3. Evaluate for source of infection (e.g., pneumonia, bacteremia, post-surgical infection)
4. Test for antimicrobial susceptibility, particularly methicillin resistance
5. Monitor response to therapy with clinical assessment and follow-up imaging

Common Pitfalls to Avoid

1. Dismissing S. aureus as a contaminant without considering clinical context
2. Failing to obtain adequate sample volume for complete pleural fluid analysis
3. Not considering S. aureus bacteremia as a source of pleural infection
4. Delaying appropriate antimicrobial therapy while awaiting additional test results
5. Inadequate drainage of infected pleural fluid

Conclusion

When S. aureus is isolated from a thoracentesis sample, it should be considered a true pathogen until proven otherwise, especially when found in significant quantities and accompanied by clinical or laboratory evidence of infection. Unlike respiratory specimens where S. aureus can represent colonization, its presence in pleural fluid typically indicates pathogenic involvement requiring appropriate antimicrobial therapy and possible drainage.