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Stool Gram stain

Definition

A stool Gram stain is a laboratory test that uses different stains to detect and identify bacteria in a stool sample.

The Gram stain method is sometimes used to rapidly diagnose bacterial infections.

Alternative Names

Gram stain of stool; Feces Gram stain

How the test is performed

You will need to collect a stool sample.

There are many ways to collect a sample. You can catch the stool on plastic wrap that is loosely placed over the toilet bowl and held in place by the toilet seat, and then place the sample in a clean container. (One test kit supplies a special toilet tissue that you use to collect the sample.) Do not take stool samples from the toilet bowl water, because this can cause errors.

If you need to collect a sample from a child still in diapers, line the diaper with plastic wrap. Position the plastic wrap so that it keeps the stool from any urine. Mixing of urine and stool can spoil a good sample.

Your health care provider will give you instructions on when and how to return the sample.

The sample is sent to a laboratory. A small amount is spread in a very thin layer on a glass slide. This is called a smear. A series of special stains are added to the sample. The lab team member examines the stained smear under the microscope, looking for bacteria. The color, size, and shape of the cells help identify the specific bacteria.

How the test will feel

A lab smear is painless and does not involve the patient.

There is no discomfort when a stool sample is collected at home because it only involves normal bowel functions.

Why the test is performed

Your doctor may order this test to help diagnose an intestinal infection or illness, sometimes involving diarrhea.

Normal Values

A normal result means only normal or "friendly" bacteria were seen on the stained slide. Everyone has friendly bacteria in their intestines.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

An abnormal result means that an intestinal infection may be present. Stool cultures and other tests can also help diagnose the cause of the infection.

What the risks are

There are no risks.

References

DuPont HL. Approach to the patient with suspected enteric infection. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 305.

Steiner TS, Guerrant RL. Principles and syndromes of enteric infection. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 93.