Gram stained control slides must be verified before examining and reporting patient smears. Manufacturer or user prepared QC slides can be used. If preparing your own QC slides, 18- to 24-hour cultures of known gram-positive and gram-negative organisms are used. The manufacturer’s product insert will state the organism name and American Type Culture Collection (ATCC) strain number that is recommended. This is commonly:
- Positive control: Staphylococcus aureus (should stain blue to dark purple/gram-positive cocci)
- Negative control: Escherichia coli (should stain pink to red/gram-negative rods)
Control slides serve as a QC mechanism for staining reagents and techniques. If these slides are not stained properly, the problem must be resolved and new slides made before patient specimens are examined and reported.