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 Gram stain manufacturer’s product insert will state the organism name as well as the 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, corrective action documented, and new slides made before patient specimens are examined and reported.
