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Microbiology

Quantitative Reporting

The lower respiratory tract specimens have specific requirements for reporting, especially in the context of bacterial burden and clinical relevance. Quantitative and semiquantitative reporting are essential in order to provide useful information to the healthcare team

General Principles

  • Clinical Correlation: The laboratory must provide an accurate report, which must be interpreted alongside the patient’s clinical presentation
  • Purpose of Reporting: To guide the clinical team in the management of lower respiratory tract infections, particularly pneumonia
  • Reporting Formats: The reporting of culture results from lower respiratory specimens relies on:
    • Qualitative: Positive or negative
    • Quantitative: Reporting bacterial counts
    • Semi-quantitative: Reporting based on estimated growth

Quantitative Reporting

  • Specimens Primarily Using Quantitative Reporting
    • Bronchoalveolar lavage (BAL)
    • Protected specimen brush (PSB)
    • Protected brush catheter
  • Advantages
    • More objective assessment of bacterial load
    • Helps differentiate between colonization and infection
    • Useful for monitoring treatment response
  • Procedure
    • Serial Dilutions: The specimen is diluted in a known volume of sterile saline
    • Plating: A specific volume of each dilution is plated on appropriate culture media
    • Colony Counting: After incubation, colonies are counted from the dilutions. The number of colonies is multiplied by the dilution factor to determine the colony-forming units per milliliter (CFU/mL)
    • Interpretation: The CFU/mL values are then compared to established interpretive criteria
    • Example: If 10 µL of a 1:100 dilution yields 20 colonies, then the CFU/mL is 20 x 100 x 100 = 200,000 CFU/mL
  • Interpretive Criteria
    • The interpretation relies on established cut-offs
    • These cut-offs aid in differentiating between infection and contamination or colonization
    • The cut-off for pneumonia may be > 104 CFU/mL
  • Specific Cut-Offs
    • BAL: >10,000 CFU/mL is often used
    • PSB: >1,000 CFU/mL is often used
  • When to Use Quantitative Reporting
    • Patients on Antibiotics: Assessing the response to treatment
    • Immunocompromised Patients: Infections with Pneumocystis jirovecii or other opportunistic infections, it is important to assess bacterial load
    • Healthcare-Acquired Pneumonia: Helpful in distinguishing between colonization and infection

Semi-Quantitative Reporting

  • Primarily Used for: Sputum, and sometimes for ETA
  • Procedure
    • Colony Counting: Based on the amount of the specimen that was plated
    • Grading of Growth: The technologist examines the plates, and estimates the amount of growth of the organism (e.g., few, moderate, many)
    • Reporting of Growth
      • “Few” - 1-10 colonies
      • “Moderate” - 11-50 colonies
      • “Many” - >50 colonies
    • Reporting: The laboratory will include the organism’s name and a descriptive term
  • Advantages
    • Provides some estimation
    • Faster to perform than quantitative methods
    • Applicable to sputum samples
  • Disadvantages
    • Less precise compared to quantitative methods
    • More subjective, which increases the potential for inter-observer variability
    • Does not provide a specific bacterial load
  • Interpreting Sputum Cultures
    • Sputum samples can be prone to contamination by oral flora. This means careful interpretation is needed
    • The laboratory should include an evaluation of the sample quality to help guide interpretation
    • The presence of multiple organisms can cause confusion. If a patient has signs and symptoms of pneumonia, and a sputum culture is positive, then treatment is indicated
    • If the sputum culture is positive for a known respiratory pathogen, then treatment is indicated

Factors Influencing Interpretation

  • Clinical Context
    • Symptoms: Presence of cough, fever, chest pain, and production of purulent sputum
    • Radiographic Findings: Chest X-ray findings
    • Patient’s Risk Factors: Age, underlying medical conditions, immune status, and any recent antibiotic usage
  • Specimen Quality
    • Sputum specimens must be of adequate quality. Assessment based on the presence of neutrophils and the number of squamous epithelial cells. This is a quality control measure
    • Specimens that are too contaminated should be rejected
    • BAL, ETA, and BB are considered to be better specimens
  • Organism Characteristics
    • Pathogens: Known respiratory pathogens are considered significant
    • Opportunistic Organisms: The presence of an opportunistic organism may be significant in immunocompromised patients
  • Mixed Flora: The presence of multiple organisms complicates the interpretation

Reporting and Communication

  • Clear Reporting: The laboratory report should be clear, concise, and include both the quantitative or semi-quantitative results and the identity of the organism
  • Interpretation: The report may include an interpretive comment, if necessary, to aid in the clinical management of the patient. This may be based on hospital policy
  • Prompt Communication: Critical to communicate significant results to the clinician

Key Terms

  • Quantitative Reporting: Reporting results as colony-forming units per milliliter (CFU/mL)
  • Semi-quantitative Reporting: Reporting results based on estimated growth (e.g., few, moderate, many)
  • Bronchoalveolar Lavage (BAL): A procedure to collect fluid from the lungs
  • Protected Specimen Brush (PSB): A protected brush catheter used to collect specimens
  • Colony-Forming Units (CFU): A measure of the number of viable bacteria in a sample
  • Sputum: Secretions from the lower respiratory tract that are expectorated
  • Endotracheal Aspirate (ETA): Secretions collected through an endotracheal tube
  • Pneumonia: Lung infection
  • Immunocompromised: Having a weakened immune system
  • Colony Morphology: The visual characteristics of bacterial colonies on culture media (e.g., size, shape, color, texture)
  • Patient’s Clinical Presentation: Refers to the patient’s symptoms, physical examination findings, and other relevant clinical information
  • Oral Flora: The normal bacterial community in the mouth
  • Neutrophils: A type of white blood cell
  • Squamous Epithelial Cells: Cells from the mouth and throat
  • Pathogen: An organism that causes disease
  • Opportunistic Organism: An organism that causes disease primarily in a host with a weakened immune system