From the Guidelines
When a pus sample arrives at a pathology lab for routine microscopy examination, it is crucial to process the sample promptly, but according to the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM), the specimen of choice is a biopsied sample of the advancing margin of the lesion, and pus alone is inadequate and does not represent the disease process 1.
Key Considerations for Processing Pus Samples
- The laboratory should not use the label “wound” alone and should be specific about the body site and type of wound.
- The specimen should be transported to the laboratory in a sterile container at room temperature, immediately, to ensure optimal recovery of pathogens.
- Upon receipt, the sample should be divided for both direct microscopic examination and culture, with preparation of smears for Gram staining and special stains if needed, as well as inoculation onto appropriate culture media.
Importance of Proper Specimen Handling
- Proper documentation of the sample details, including collection site, time, and patient information, is essential for accurate identification of pathogens and guiding appropriate treatment.
- The laboratory should be notified if specific pathogens, such as Nocardia spp, are suspected, so that special media can be used and routine culture plates can be incubated for an extended period.
- Antimicrobial susceptibility testing should follow any significant bacterial growth to guide antibiotic therapy, ensuring effective treatment of infections and preventing complications from untreated or improperly treated infections 1.
From the Research
Routine Microscopy Examination of Pus Samples
If a pus sample comes for routine microscopy examination in a pathology lab, the following steps should be taken:
- The sample should be subjected to Gram staining to differentiate between Gram-positive and Gram-negative bacteria 2, 3, 4.
- The Gram stain technique involves the use of a crystal violet-iodine complex and a safranin counterstain to distinguish between Gram-positive and Gram-negative bacteria 3.
- The cell walls of Gram-positive organisms retain the crystal violet-iodine complex and appear purple, whereas Gram-negative organisms decolorize and appear pink 3.
- A modified Gram stain technique can be used to enhance the contrast between bacteria and host tissue, especially in tissue samples 5.
- The modified Gram stain technique can clearly identify both Gram-positive and Gram-negative bacteria and provide excellent contrast between bacteria and non-viable tissue 5.
- In addition to Gram staining, other diagnostic tools such as the Accelerate Pheno™ system can be used for rapid identification and antimicrobial susceptibility testing of Gram-positive and Gram-negative bacteria 6.