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The page below is a sample from the LabCE course Reading Gram Stained Smears From Cultures. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Gram Stain Quality Control (QC) Organisms

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.