Since the primary reason for obtaining a sputum culture in a patient with pneumonia is to sample the lower respiratory tract, you should first verify that the sputum was “adequate” by reviewing the gram stain. Absence of neutrophils (unless the patient is neutropenic) with or without epithelial cells on gram stain of sputum suggests that it may not be an adequate sample (ie, likely saliva)¹, and therefore growth of normal respiratory flora (NRF) should not be surprising in this setting.  

Other potential explanations for NRF on sputum culture in patients with CAP include:^2-5

• Delay in sputum processing with possible overgrowth of oropharyngeal flora.
• Pneumonia caused by pathogens that do not grow on standard sputum culture media (eg, atypical organisms, viruses, anaerobes).
• Pneumonia caused by potential pathogens such as as Streptococcus mitis and Streptococcus anginosus group that may be part of the NRF.
• Initiation of antibiotics prior to cultures (eg, in pneumococcal pneumonia).

Of note, since 2010, several studies have shown that over 50% of patients with CAP do not have an identifiable cause.³ So, growing NRF from sputum of patients with CAP appears to be common.

Liked this post? Download the app on your smart phone and sign up below to catch future pearls right into your inbox, all for free!

References

1. Wong LK, Barry AL, Horgan SM. Comparison of six different criteria for judging the acceptability of sputum specimens. J Clin Microbiol 1982;16:627-631. https://www.ncbi.nlm.nih.gov/pubmed/7153311
2. Donowitz GR. Acute pneumonia. In Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases (2010). Churchill Livingstone, pp 891-916.
3. Musher DM, Abers MS, Bartlett JG. Evolving understanding of the causes of pneumonia in adults, with special attention to the role of pneumococcus. Clin Infect Dis 2017;65: 1736-44. https://www.ncbi.nlm.nih.gov/pubmed/29028977
4. Abers MS, Musher DM. The yield of sputum culture in bacteremic pneumococcal pneumonia after initiation of antibiotics. Clin Infect Dis 2014; 58:1782. https://www.ncbi.nlm.nih.gov/pubmed/24604901
5. Bartlett JG, Gorbach SL, Finegold SM. The bacteriology of aspiration pneumonia. Bartlett JG, Gorbach SL, Finegold SM. Am J Med 1974;56:202-7. https://www.ncbi.nlm.nih.gov/pubmed/4812076