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Related theory

Excessive respiratory secretions may be due to increased mucus production in cases of infection, impaired mucociliary transport or a weak cough reflex. Lower airway secretions that are not cleared provide an ideal medium for bacterial growth. Suitable microbiological analysis in diagnosing infection will depend upon (PHE [191]):
  • adequacy of lower respiratory tract specimens
  • avoidance of contamination by upper respiratory tract and oral flora
  • use of microscopic techniques and culture methods
  • current and recent antimicrobial therapy.
Sputum is a combination of mucus, inflammatory and epithelial cells, and degradation products from the lower respiratory tract. It is never free from organisms since material originating from the lower respiratory tract has to pass through the pharynx and the mouth, which have commensal populations of bacteria (Keogh [114], Pagana and Pagana [177], PHE [191]). However, it is important to ensure that material sent to the microbiology laboratory is sputum rather than a saliva sample, which will contain squamous epithelial cells and be unrepresentative of the underlying pulmonary pathology.
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