Gram Stain: MedlinePlus Medical Test

What do the results mean?

Your sample will be placed on a slide and treated with the Gram stain. A laboratory professional will examine the slide under a microscope. If no bacteria were found, it means you probably don’t have a bacterial infection or there weren’t enough bacteria in the sample.

If bacteria were found, it will have certain qualities may provide important information about your infection:

  • If the bacteria was colored purple, it means you likely have a Gram-positive infection.
  • If the bacteria was colored pink or red, it means you likely have a Gram-negative infection.

Your results will also include information about the shape of the bacteria in your sample. Most bacteria are either round (known as cocci) or rod-shaped (known as bacilli). The shape can provide more information about the type of infection you have.

Although your results may not identify the exact type of bacteria in your sample, they can help your provider get closer to finding out what’s causing your illness and how best to treat it. You may need more tests, such as a bacteria culture, to confirm what type of bacteria it is.

Gram stain results may also show whether you have a fungal infection. The results may show what category of fungal infection you have: yeast or mold. But you may need more tests to find out which specific fungal infection you have.

If you have questions about your results, talk to your health care provider.

Learn more about laboratory tests, reference ranges, and understanding results.