Which of the following specimen types is likely to have the LEAST amount of bacterial contamination with oral flora?

Question

BlackTom AI · Accepted Answer

Bronchoalveolar lavage samples are obtained from the lower respiratory tract via bronchoscopy and therefore are least contaminated by oral flora compared with specimens collected from the mouth or pharynx.

1. Relevant knowledge points and concepts
- Oral flora: the microbiota resident in the mouth and oropharynx (saliva, tonsils, throat) that readily contaminate specimens passing through the oral cavity.  
- Specimen collection sites: saliva and throat swabs sample the oral/pharyngeal mucosa directly; expectorated sputum passes through the mouth and is frequently contaminated; bronchoalveolar lavage (BAL) samples the alveolar/lower airway spaces via a bronchoscope and is designed to sample the lower respiratory tract with minimal upper airway contamination.  
- Clinical microbiology principle: the more distal (lower respiratory) the sample and the more protected the collection method, the less likely it is to contain oral commensals.

2. Step-by-step reasoning
- Saliva: directly composed of oral secretions and commensal bacteria → highest oral flora.  
- Throat swab: taken from oropharynx → collects mucosal flora and pathogens but also abundant oral commensals.  
- Sputum: expectorated material originates in lower airways but must pass through mouth, picking up oral flora; quality criteria (few squamous epithelial cells) are used to judge contamination because sputum often is contaminated.  
- Bronchoalveolar lavage: obtained by instilling and retrieving fluid in a distal airway segment through a bronchoscope; technique reduces contamination from the oropharynx relative to the others.

3. Calculations
- None required for this comparative question.

4. Why the correct answer is correct
- BAL samples the lower respiratory tract using a protected, invasive technique, so it presents the least amount of oral flora contamination among the listed specimens.

5. Why the other options are incorrect
- Saliva: composed almost entirely of oral flora—high contamination.  
- Throat swab: directly samples oropharyngeal mucosa—high contamination.  
- Sputum: expectorated through the mouth and commonly contaminated by oral commensals; even good-quality sputum may have more contamination than BAL.

6. Conclusion
- Therefore, Bronchoalveolar lavage is the specimen type likely to have the least bacterial contamination with oral flora. This matches the provided correct answer: Bronchoalveolar lavage.

Which of the following specimen types is likely to have the LEAST amount of bacterial contamination with oral flora?单项选择题

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