Gram stained control slides must be checked before examining patients' smears. The positive control, Staphylococcus aureus, should stain blue and the negative control, Escherichia coli, should stain pink. Control slides serve as a quality control mechanism for staining reagents and techniques. Control slides must be prepared daily and when a new bottle/lot # of stain is used.

The macroscopic appearance of correctly stained control slides is shown in the two images on the right: The upper image show a correctly stained Gram positive slide, and the lower image shows a correctly stained Gram negative slide.