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Gram Stain

What is a gram stain?

If your doctor suspects you have an infection, they may order a culture and gram stain to check for bacteria. If bacteria are present, this test can also help your doctor learn if the bacteria are gram negative or gram positive. The difference between gram-negative and gram-positive bacteria can affect their recommended treatment plan.

Gram stains can be performed on various types of specimens, including:

  • blood
  • tissue
  • stool
  • urine
  • sputum

What is a gram stain used for?

Your doctor may order a gram stain if you have symptoms of an infection. They may not know if the infection is bacterial, viral, fungal, or parasitic. These types of infections are typically treated differently. Different types of bacterial infections may also require different treatments.

Your doctor can use a gram stain to learn if bacteria are responsible for your symptoms and what types of bacteria are present. They may also order other tests to help diagnose your illness.

How is a gram stain administered?

In order to perform a gram stain, your doctor will need to collect a sample of body fluid or tissue for analysis. Their collection methods will vary depending on the type of sample they need. For example, to collect a sample of sputum, they may ask you to cough some into a specimen container. To collect a sample of urine, they may ask you urinate into a sample cup. To collect a sample of blood, they may perform a blood draw.

After they have collected the sample, they will send it to a laboratory for testing. A technician will use a special staining technique to make it easier to see bacteria under a microscope.

What do the results of a gram stain mean?

If your gram stain results are negative, it means no bacteria were found in your sample. If they’re positive, it means bacteria were present.  

Because of the staining technique used, gram-positive bacteria will appear purple under a microscope and gram-negative bacteria will appear pink. The shape, size, and quantity of bacteria present will also provide information about your infection.

What are the risks of a gram stain?

There are no known risks involved in performing a gram stain on a sample of urine, stool, or mucus. If your doctor orders a gram stain on a sample of blood or tissue, some risks are involved in collecting the sample. For example, you may experience bruising, bleeding, or infection as a result of a blood draw. Ask your doctor for more information about the specific risks involved in your case.

What are the benefits of a gram stain?

The main benefit of a gram stain is that it helps your doctor learn if you have a bacterial infection, and it determines what type of bacteria are causing it. This can help your doctor determine an effective treatment plan.

What is your outlook following a gram stain?

Your treatment plan and outlook will vary depending on your diagnosis.

If you’re diagnosed with a bacterial infection, your doctor may prescribe antibiotics. If your gram stain is negative for disease-causing bacteria, your doctor may order other tests to learn what’s causing your symptoms.  

Content licensed from:

Written by: MaryAnn DePietro
Medically reviewed on: Oct 07, 2016: University of Illinois-Chicago, College of Medicine

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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