Sputum specimens
Before culturing a sputum specimen, a Gram stain should be performed to evaluate the quality of the specimen. Two criteria are currently used to determine if the specimen is contaminated with oral flora organisms (which would make the specimen unsuitable for culture).
Laboratories should choose one of the following:
- One criterion states that the sputum specimen should be rejected if 25 or more squamous epithelial cells/low power field (SQE/LPF) are observed.
- The second criterion recommends a cutoff of more than 10 SQE/LPF.
- In either case, a minimum of 20 LPFs should be observed.
Review and follow your specific laboratory's criteria. (One exception to rejection: never reject a sputum sample from a cystic fibrosis patient.)
The low-power microscopic field on the right is representative of 20 microscopic fields that were reviewed in this Gram-stained preparation of sputum. This specimen would be considered unacceptable for culture.
If the specimen is determined to be a sub-optimal specimen, the clinician should be contacted and a request should be made for a new specimen. It is important to communicate that culturing the specimen that was provided will not yield useful information about the possible pathogens from the lower respiratory tract.
If the specimen is determined to be a good quality lower respiratory tract specimen, continue to examine the slide under oil immersion (1000x) magnification for bacteria, yeast, and polymorphonuclear white blood cells (PMNs) and proceed with culturing the specimen.
