According to regulatory requirements1, Gram stain quality control (QC) testing must be performed with each new batch of stain and at least weekly, using both gram-positive and gram-negative organisms. However, individual laboratories may have more stringent requirements. No matter what the required frequency for QC testing - if you prepare and read Gram stains infrequently - it would be wise to stain quality control slides at the same time as a patient Gram stain smear.
The image on the right represents control slides using Staphylococcus aureus as the gram-positive control and Escherichia coli as the gram-negative control.
- If properly stained, the Staphylococcus aureus on the positive control slide (side A in the image) will stain blue, or gram-positive.
- The Escherichia coli on the negative control slide (side B in the image) will stain red, or gram-negative.
- If these slides are not stained properly, the problem must be resolved, corrective actions documented and new slides made before patient specimens are examined and reported.
It is also helpful to maintain a set of Gram stain reference slides for competency training and testing. Knowledge of Gram reaction and organism morphology should be assessed, as well as knowledge regarding troubleshooting the Gram stain.
