Gram stain of stool

Alternative names 
Stool Gram stain; Feces Gram stain

Definition

Gram stain is a method of staining microorganisms (bacteria) using a special series of stains. In this test, a specimen of stool is stained and then examined under the microscope.
The Gram stain method can be applied to almost any clinical specimen and is one of the most commonly used techniques for the rapid diagnosis of bacterial infections.

How the test is performed

A small amount of stool is applied in a very thin layer to a microscope slide. This is called a smear. A series of stains called a Gram stain is applied to the specimen. It is first stained with crystal violet stain, then iodine, then decolorized, then stained with safranin.

The stained smear is then examined under the microscope for the presence of bacteria. The color, size, and morphologic appearance (shape) of the cells help identify the infecting organism.

How to prepare for the test
Adults and children:
There are many ways to collect the samples. You can catch the stool on plastic wrap that is loosely placed over the toilet bowl and held in place by the toilet seat. Then, put the sample in a clean container. One test kit supplies a special toilet tissue that you use to collect the sample, then put the sample in a clean container.

Infants and young children:
For children wearing diapers, line the diaper with plastic wrap. If the plastic wrap is positioned properly, isolating the stool from any urine output, mixing of urine and stool can be prevented for a better sample.

How the test will feel
Some people feel discomfort or embarrassment with this test.

Why the test is performed
The test may be performed to try to diagnose an infection or illness, sometimes involving diarrhea.

Normal Values
The presence of only normal or “friendly” organisms is normal. All humans have bacteria in their intestines.

What abnormal results mean
An intestinal infection may be present. Stool cultures and more sophisticated tests can also help diagnose the cause of the infection.

What the risks are
There are no risks.

Special considerations
Additional examinations of the specimen may be indicated for conditions other than infection.

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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