Question

The most appropriate specimen for routine microbiological examination of lower respiratory tract infection is :

• A. Saliva collected after rinsing the mouth
• B. Deep-cough early morning sputum
• C. Nasal Secretion
• D. Oral Swab

Answer 1

Answer:

B. Deep-cough early morning sputum

Specimen Selection for Lower Respiratory Tract Infections (LRTI)

• Sputum is considered the gold standard for routine microbiological examination in cases of suspected lower respiratory tract infections, such as bacterial pneumonia.
• Quality Criteria: To be considered a valid diagnostic specimen, a sputum sample must originate from the lower respiratory tract rather than the oropharynx. Laboratories typically screen samples using Gram staining; specimens are deemed acceptable for culture only if they contain >25 neutrophils and <10 squamous epithelial cells per low-power field (10x magnification).
• Collection Procedure: The ideal specimen is an early morning deep cough sample. Patients should be instructed to rinse their mouths with water before collection to minimize contamination from commensal oral flora.
• Alternative Specimens: In patients unable to produce sputum, alternative invasive procedures are required to obtain high-quality lower respiratory secretions, including:
  • Endotracheal Aspirates (ETA): Primarily used for ventilated patients.
  • Bronchoalveolar Lavage (BAL): Utilized for deep-seated infections or immunocompromised patients.
  • Induced Sputum: Collected via nebulization with hypertonic saline for patients with non-productive cough.
• Limitations: Saliva and nasopharyngeal secretions are unacceptable samples as they reflect the upper respiratory flora and yield misleading culture results. Rapid transport to the laboratory is essential to maintain the viability of fastidious pathogens like Streptococcus pneumoniae or Haemophilus influenzae.