Health Encyclopedia

Gram Stain

Does this test have other names?

Gram stain procedure

What is this test?

This test looks at bodily fluids to find out whether you have a bacterial infection—and if so, which kind. A gram stain can be used on sputum, blood, urine, and the fluid in your spine and joints.

If you have a bacterial infection, your fluid sample will have bacteria in it that are either gram-positive or gram-negative. Either can cause illness. The gram stain test will cause the bacteria to turn certain colors. This will help your healthcare provider find out which type of bacteria is causing your infection.

A few organisms can be either gram-positive or gram-negative. 

Why do I need this test?

You may need this test if your healthcare provider suspects that you have a bacterial infection but needs to find out what type of bacteria is causing it.

What other tests might I have along with this test?

Depending on what's bothering you, your healthcare provider may order a series of different tests for signs of a bacterial infection. If you have a lung infection, your provider may order tests such as a chest X-ray to see how well your lungs are working. You may also have a complete blood count. 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.  

Results will include the color of your gram stain, whether your sample was gram-positive or gram-negative, and the shape of the bacteria. A gram stain can't always give the type of bacteria causing an illness, but it can often help your healthcare provider make a diagnosis. 

How is the test is done?

This test requires a sample of a bodily fluid or tissue. Which fluid or tissue depends on what kind of illness you have.

For an intestinal infection, for example, your healthcare provider may ask for a stool sample. If you are being tested for bacterial pneumonia, you will likely have to give a sample by coughing and producing mucus. 

Does this test pose any risks?

This test poses no known risks for a stool, urine, or sputum sample. Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward the site may be slightly sore.

Talk with your healthcare provider about the possible risks involved in obtaining a sample if the collection procedure is an invasive one, like draining an infection or testing for meningitis. 

What might affect my test results?

Taking prescription antibiotics may result in a false-negative.

How do I get ready for this test?

Ask your healthcare provider how you should prepare if you need to have a fluid sample taken from your spine or joints.

If you have to cough to get mucus from your lungs, you may have to take extra measures when you give a sample. Your provider may suggest that you stop eating 1 to 2 hours before the test. You may also have to rinse your mouth before coughing to give a sample.  

Medical Reviewers:

  • Taylor, Wanda L, RN, PhD
  • Ziegler, Olivia Walton, MS, PA-C