Gram Stain
Reviewed
Also called: Gram's stain, Gram stain test
A Gram stain is a test that checks to see if you have a bacterial infection.
Specimen: Varies by infection site: sputum, wound drainage, urine, blood, or fluid from joints, heart, lungs, or spinal cord
Why this test may be ordered
- To find out if you have a bacterial infection
- To determine whether the bacteria are Gram-positive or Gram-negative
- To help your provider decide what other tests are needed and how to treat the infection
- May also be used to diagnose fungal infections
Preparation and collection
You don't need any special preparations for a Gram stain.
Your provider will take a sample from the site of a suspected infection or from certain body fluids, depending on what type of infection you may have. After your provider collects your sample, they will be sent to a laboratory.
There is no risk to having a swab, sputum, or urine test. There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Understanding results
Your sample will be placed on a slide and treated with the Gram stain. A laboratory professional will examine the slide under a microscope.
- No bacteria found: If no bacteria were found, it means you probably don't have a bacterial infection, or there weren't enough bacteria in the sample. Limitation: A Gram stain cannot detect a virus. Next step: Your provider will consider your symptoms, and medical history to understand the results of a Gram stain test.
- Bacteria colored purple (Gram-positive): If the bacteria was colored purple, it means you likely have a Gram-positive infection. Limitation: Your results may not identify the exact type of bacteria in your sample, but they can help your provider figure out what's causing your illness and how best to treat it. Next step: You may need more tests, such as a bacteria culture, to confirm what type of bacteria it is.
- Bacteria colored pink or red (Gram-negative): If the bacteria was colored pink or red, it means you likely have a Gram-negative infection. Limitation: Your results may not identify the exact type of bacteria in your sample, but they can help your provider figure out what's causing your illness and how best to treat it. Next step: You may need more tests, such as a bacteria culture, to confirm what type of bacteria it is.
- Fungal infection (yeast or mold): Gram stain results may also show whether you have a fungal infection. Limitation: But you may need more tests to find out which specific fungal infection you have. Next step: They may also want you to have a physical exam or additional tests.
- A Gram stain cannot detect a virus.
- Results may not identify the exact type of bacteria; a bacteria culture may be needed to confirm.
- If no bacteria are found, it may mean there weren't enough bacteria in the sample.
What to do next
If you are diagnosed with a bacterial infection, you will probably be prescribed antibiotics. It's important to take your medicine as prescribed, even if your symptoms are mild.
- What do my Gram stain results show?
- Do I need any additional tests?
- What type of infection do I have?
- What treatment do you recommend?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.