Question
Discuss laboratory diagnosis of LRTI.
Answer
Laboratory diagnosis of Lower Respiratory Tract Infection (LRTI)
1. Specimen collection: sputum (expectorated, or induced if the patient cannot produce a sample spontaneously), endotracheal aspirate/bronchoalveolar lavage (BAL) in ventilated/intubated patients, and blood cultures (useful in bacteraemic pneumonia).
2. Specimen quality assessment: a good-quality sputum sample should show, on Gram stain, >25 pus cells and <10 squamous epithelial cells per low-power field (by standard criteria, e.g., Bartlett’s/Murray-Washington grading) — a sample dominated by squamous cells indicates oral contamination (predominantly saliva) rather than a genuine lower respiratory sample, and should be rejected/a fresh sample requested.
3. Direct microscopy:
- Gram stain — rapid presumptive identification of predominant organism morphology (e.g., Gram-positive diplococci suggesting Streptococcus pneumoniae).
- Ziehl-Neelsen (acid-fast) stain — for suspected pulmonary tuberculosis, detecting acid-fast bacilli.
- KOH mount — if fungal pneumonia is suspected.
4. Culture:
- Sputum/BAL culture on blood agar, chocolate agar (for fastidious organisms like Haemophilus influenzae), and MacConkey agar, with identification and antimicrobial susceptibility testing of significant isolates.
- Blood culture — positive in a proportion of bacterial pneumonia cases, particularly pneumococcal pneumonia, and is not subject to the contamination issues affecting sputum.
- Mycobacterial culture (Lowenstein-Jensen medium or liquid culture systems, e.g., BACTEC/MGIT) — for suspected tuberculosis, alongside rapid molecular tests (e.g., CBNAAT/GeneXpert MTB/RIF).
5. Antigen detection tests:
- Urinary antigen tests — for Streptococcus pneumoniae and Legionella pneumophila, offering rapid results independent of prior antibiotic therapy.
- Rapid antigen tests for respiratory viruses (influenza, RSV).
6. Molecular methods: Multiplex PCR respiratory panels, detecting a broad range of bacterial and viral pathogens simultaneously with high sensitivity, increasingly used particularly in severe/atypical or hospital-acquired pneumonia.
7. Serology: for atypical pathogens (e.g., Mycoplasma pneumoniae, Chlamydophila pneumoniae), IgM/IgG antibody testing or paired acute-convalescent titres.
8. Radiological correlation: chest X-ray/CT findings support but do not by themselves establish microbiological diagnosis; used alongside laboratory results for overall clinical assessment.