Gram Stain Test
A Gram stain is a test that checks for bacterial infection. It shows whether bacteria are Gram-positive (purple) or Gram-negative (pink or red), which helps your provider choose effective antibiotics.
What this test measures
A Gram stain checks for bacteria in a sample from a suspected infection site, such as throat, lungs, genitals, skin wound, blood, or urine. It groups bacteria by color (Gram-positive or Gram-negative) and shape to help identify the infection type.
What your results mean
Reference ranges vary by laboratory and by your age and sex. MedlinePlus does not publish a single normal range for this test — always read your result against the range printed on your own lab report and discuss it with your provider.
Frequently asked questions
What does a Gram stain test for?
It checks for bacterial infection and shows whether bacteria are Gram-positive or Gram-negative.
How is the Gram stain performed?
A sample is taken from the infection site, stained purple, then a solvent is added. Bacteria either stay purple (Gram-positive) or turn pink/red (Gram-negative).
What do the colors mean?
Purple means Gram-positive bacteria; pink or red means Gram-negative bacteria.
Can a Gram stain diagnose fungal infections?
Yes, it may show if you have a fungal infection and whether it is yeast or mold, but more tests are often needed.
Do I need other tests after a Gram stain?
You may need more tests, such as a bacteria culture, to confirm the exact type of bacteria.
Related lab tests
Sources
- MedlinePlus: Gram Stain — NIH MedlinePlus
Last updated . Information is aggregated from official public sources and is not a substitute for professional medical care.