The quality of an expectorated sputum is evaluated based on which criterion?

• A. Amount of bacteria seen in a direct Gram stain
• B. Number of red blood cells in comparison to epithelial cells in a direct Gram stain
• C. Number of white blood cells in comparison to epithelial cells in a direct Gram stain
• D. Viscosity of the sample and macroscopic appearance of blood

Answer: (C)

In evaluating expectorated sputum, you’re looking for how representative the sample is of the lower respiratory tract. The best indicator is the ratio of white blood cells to squamous epithelial cells seen in a direct Gram stain. A high white blood cell count relative to epithelial cells means the specimen is more likely from the lower airways and is of good quality for culture. If there are many epithelial cells and few white blood cells, the sample is likely contaminated with saliva and is of poor quality, even if bacteria are present.

Red blood cells aren’t used to gauge sputum quality—they can appear for other reasons and don’t indicate where the sample originated. The viscosity or macroscopic appearance of blood isn’t a standardized measure of quality either. Likewise, the amount of bacteria seen on Gram stain isn’t the criterion for quality, because oral flora can appear in contaminant samples and doesn’t alone determine sample provenance.