When to Consider Bacterial Contamination in Bronchoalveolar Lavage
Bacterial contamination should be suspected in BAL when organisms are recovered at concentrations <10⁴ cfu/mL, when typical oropharyngeal flora (enterococci, viridans streptococci, coagulase-negative staphylococci, Candida species) are isolated, or when there is poor correlation between Gram stain findings and clinical presentation. 1
Quantitative Thresholds for Distinguishing Contamination from Infection
Standard Diagnostic Cutoffs
- BAL fluid threshold: Bacterial concentrations ≥10⁴ cfu/mL are generally considered diagnostic of true infection, while lower concentrations typically represent contamination or colonization 1
- Endotracheal aspirate threshold: ≥10⁶ cfu/mL is required for diagnosis, with sensitivity of 38-82% and specificity of 72-85% 1
- Protected specimen brush: ≥10³ cfu/mL is diagnostic 1
Normal Subject Data
- BAL fluid from healthy individuals is frequently contaminated by oropharyngeal flora but typically yields <10⁴ cfu/mL 2
- In normal subjects, 7 of 8 BAL specimens showed 1-4 bacterial strains, but all were below the diagnostic threshold 2
- Protected brush catheter specimens from normal subjects are usually sterile (7 of 8 specimens) or yield <10³ cfu/mL 2
Organisms That Typically Represent Contamination
Organisms to Disregard
The following organisms should rarely if ever be considered causative pathogens in respiratory specimens: 1
- Enterococci
- Viridans streptococci
- Coagulase-negative staphylococci
- Candida species 1
Organisms Requiring Careful Interpretation
Potential bacterial pathogens that may represent either contaminants or true pathogens include: 1
- Pseudomonas aeruginosa
- Enterobacteriaceae
- Streptococcus pneumoniae
- Staphylococcus aureus
- Haemophilus influenzae
The distinction between pathogen and colonizer is facilitated by: 1
- Detection of organisms as dominant flora on direct Gram stain
- Recovery in moderate or heavy growth (≥10⁴ cfu/mL for BAL)
- Correlation with clinical presentation
Impact of Prior Antibiotic Therapy
Effect on Bacterial Counts
- Prior antibiotic administration significantly lowers quantitative bacterial counts: 1
Adjusting Interpretation
- Lower the diagnostic threshold if the patient has recently changed antibiotic therapy or if clinical probability of infection is high 1
- Patients started on therapy within 24-72 hours may have negative cultures despite true infection 1
Specimen Quality Assessment
Gram Stain Criteria
BAL specimens should be evaluated for adequacy before interpretation: 1
- Results should only be considered when there are >25 polymorphonuclear cells per high-power field 1
- Specimens with >10 squamous epithelial cells per high-power field suggest significant oropharyngeal contamination 1
- Only 15% of endotracheal aspirates meet strict quality criteria 1
Culture Interpretation
Results should be considered valid only when: 1
- Pure culture of a single microbial agent is present, OR
- A microorganism is present in an amount >10⁷ cells/mL 1
Sources of Contamination
Upper Airway Contamination
- Bronchoscope inner channel aspirates are characteristically contaminated by upper respiratory flora 1
- Saline instillation in endotracheal tubes may introduce pathogens from tube biofilm or upper airway into lower airways 1
- BAL fluid from normal subjects frequently shows oropharyngeal bacterial flora at low concentrations 2
Technical Considerations
- Deep tracheal suctioning without saline is preferred when adequate specimens can be obtained 1
- Nonbronchoscopic BAL (mini-BAL) can reliably obtain lower respiratory secretions while minimizing contamination 1
Clinical Algorithm for Interpretation
Step 1: Assess Specimen Quality
- Verify adequate polymorphonuclear cells (>25/HPF) and minimal squamous cells (<10/HPF) 1
- Confirm appropriate specimen collection technique was used 1
Step 2: Review Quantitative Culture Results
- ≥10⁴ cfu/mL in BAL: Consider true pathogen 1
- <10⁴ cfu/mL in BAL: Likely contamination or colonization 1, 2
- Adjust threshold downward if recent antibiotic use 1
Step 3: Correlate with Gram Stain
- Organisms seen as dominant flora on Gram stain support true infection 1
- Discordance between Gram stain and culture suggests contamination 1
- Only 28% concordance exists between positive Gram stain and urinary antigen tests, highlighting the need for multiple diagnostic modalities 1
Step 4: Identify Organism Type
- Disregard: Enterococci, viridans streptococci, coagulase-negative staphylococci, Candida 1
- Interpret cautiously: Common respiratory pathogens that may colonize 1
- Consider pathogenic: Organisms rarely found as colonizers (Legionella, Pneumocystis) 1
Step 5: Consider Clinical Context
- Assess pretest probability of pneumonia 1
- Review timing of antibiotic administration 1
- Evaluate severity of illness and clinical stability 1
Common Pitfalls to Avoid
Overinterpretation of Low-Level Growth
- Do not treat organisms below diagnostic thresholds unless clinical suspicion is extremely high and patient is deteriorating 1
- Remember that BAL from normal subjects frequently shows low-level bacterial growth 2
Ignoring Antibiotic Effect
- Recent antibiotic use (within 72 hours) dramatically reduces sensitivity of quantitative cultures 1
- Consider alternative diagnostic methods (urinary antigens, serology) when antibiotics have been administered 1
Treating Colonizers
- Avoid treating typical contaminants (enterococci, coagulase-negative staphylococci, Candida) even if present in moderate amounts 1
- Focus on clinically relevant pathogens that correlate with presentation 1