gram-stain
Reviewed
Also called: Gram's stain, Gram stain test
A Gram stain is a test that checks for a bacterial infection. A sample from the suspected infection site is stained purple, then a solvent is added. Bacteria that stay purple are Gram-positive; bacteria that turn pink or red are Gram-negative. The test helps identify the type of infection and guide antibiotic choice.
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. Common collection methods include: a special swab for a wound or throat sample, a blood draw from a vein, a sterile urine sample, or coughing up sputum into a cup. For fluid around joints, heart, lungs, or spinal cord, a fine-needle aspiration biopsy may be used.
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 needle site, but most symptoms go away quickly.
Understanding results
Your sample will be placed on a slide, treated with the Gram stain, and examined under a microscope. Your provider will consider your symptoms and medical history to understand the results.
- No bacteria found: You probably don't have a bacterial infection, or there weren't enough bacteria in the sample. Limitation: A negative result does not completely rule out infection if the sample was inadequate. Next step: Your provider may recommend additional tests based on your symptoms.
- Bacteria found, colored purple (Gram-positive): You likely have a Gram-positive infection. Examples include MRSA, strep infections, and toxic shock syndrome. Limitation: The test may not identify the exact type of bacteria; a bacteria culture may be needed to confirm. Next step: Your provider will consider the shape of the bacteria (round cocci or rod-shaped bacilli) and may order additional tests to guide treatment.
- Bacteria found, colored pink or red (Gram-negative): You likely have a Gram-negative infection. Examples include salmonella, pneumonia, urinary tract infections, bacterial meningitis, and gonorrhea. Limitation: The test may not identify the exact type of bacteria; a bacteria culture may be needed to confirm. Next step: Your provider will consider the shape of the bacteria and may order additional tests to guide treatment.
- Fungal elements seen (yeast or mold): You may have a fungal infection. The Gram stain can show whether the fungus is yeast or mold. Limitation: More tests are needed to find out which specific fungal infection you have. Next step: Your provider may order additional tests to identify the fungus and determine treatment.
- 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. This can prevent your infection from getting worse and causing serious complications. If you have questions about your results, talk to your provider.
- 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?