What is a Gram stain?

A Gram stain is a test that checks for bacteria at the site of a suspected infection or in certain body fluids, such as blood or urine. These sites include the throat, lungs, and genitals, and in skin wounds.

There are two main categories of bacterial infections: Gram-positive and Gram-negative. The categories are diagnosed based on the how the bacteria reacts to the Gram stain. A Gram stain is colored purple. When the stain combines with bacteria in a sample, the bacteria will either stay purple or turn pink or red. If the bacteria stays purple, they are Gram-positive. If the bacteria turns pink or red, they are Gram-negative. The two categories cause different types of infections:

Gram-positive infections include methicillin-resistant Staphylococcus aureus (MRSA), strep infections, and toxic shock. Gram-negative infections include salmonella, pneumonia, urinary tract infections, and gonorrhea. A Gram stain may also be used to diagnose fungal infections.

Other names: Gram's stain

What is it used for?

A Gram stain is most often used to find out if you have a bacterial infection. If you do, the test will show if your infection is Gram-positive or Gram-negative.

Why do I need a Gram stain?

You may need this test if you have symptoms of a bacterial infection. Pain, fever, and fatigue are common symptoms of many bacterial infections. Other symptoms will depend on the type of infection you have and where in the body it is located.

What happens during a Gram stain?

Your health care provider will need to take a sample from the site of a suspected infection or from certain body fluids, depending on what type of infection you may have. The most common types of Gram stain tests are listed below.

Wound sample:
A provider will use a special swab to collect a sample from the site of your wound.

Blood test:
A provider will take a sample of blood from a vein in your arm.

Urine test:
You will provide a sterile sample of urine in a cup, as instructed by your health care provider.

Throat culture:
Your health care provider will insert a special swab into your mouth to take a sample from the back of the throat and tonsils.

Sputum culture. Sputum is a thick mucus that is coughed up from the lungs. It is different from spit or saliva. Your health care provider will ask you to cough up sputum into a special cup, or a special swab may be used to take a sample from your nose.

Will I need to do anything to prepare for the test?

You don't need any special preparations for a Gram stain.

Are there any risks to the test?

There is no risk of having a swab, sputum, or urine test.

There is very little risk to having a blood test. You may have slight pain or bruise at the spot where the needle was put in, but most symptoms go away quickly.