Bacterial sputum cultures. A clinician's viewpoint.

Area of Science:

• Microbiology
• Clinical Diagnostics
• Pulmonary Medicine

Background:

• Bacterial pneumonia diagnosis often relies on sputum cultures.
• The reliability of expectorated sputum versus transtracheal aspiration for bacterial culture is debated.
• Previous analyses included bacterial species not typically causing adult pneumonia.

Purpose of the Study:

• To re-evaluate bacterial culture data from expectorated sputum and transtracheal aspiration.
• To determine the optimal threshold for leukocyte count in sputum samples for bacterial culture acceptance.
• To assess the impact of excluding non-pathogenic species on culture interpretation.

Main Methods:

• Reanalysis of existing bacterial culture data from sputum and transtracheal aspirates.
• Exclusion of bacterial species unlikely to cause pneumonia in adults.
• Comparison of the number of bacterial isolates per specimen based on leukocyte counts (per field).

Main Results:

• After excluding unlikely species, the mean number of isolates per specimen was not excessive.
• Sputum samples with >25 leukocytes/field showed similar bacterial isolate numbers (1.04-1.05) compared to those with <25 leukocytes/field (0.57-0.59).
• This suggests a higher leukocyte count indicates a more reliable sample for bacterial culture.

Conclusions:

• Expectorated sputum samples containing more than 25 leukocytes per field are recommended for bacterial culture.
• This leukocyte threshold aids in differentiating true infection from contamination.
• Utilizing this criterion can improve the diagnostic accuracy of sputum cultures for pneumonia.