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Gram stain

Show Alternative Names
Urethral discharge - Gram stain
Feces - Gram stain
Stool - Gram stain
Joint fluid - Gram stain
Pericardial fluid - Gram stain
Gram stain of urethral discharge
Gram stain of the cervix
Pleural fluid - Gram stain
Sputum - Gram stain
Skin lesion - Gram stain
Gram stain of skin lesion
Gram stain of tissue biopsy

A Gram stain is a test used to identify bacteria. It is one of the most common ways to quickly diagnose bacterial infection in the body.

How the Test is Performed

How the test is performed depends on what tissue or fluid from your body is being tested. The test may be quite simple, or you may need to prepare ahead of time.

  • You may need to provide a sputum, urine, or stool sample.
  • Your health care provider may use a needle to take fluid from your body to test. This could be from a joint, from the sac around your heart, or from the space around your lungs.
  • Your provider may need to take a tissue sample, such as from your cervix or skin.

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 stains are added to the sample.
  • A 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 type of bacteria.

How to Prepare for the Test

Your provider will tell you what to do to prepare for the test. For some types of tests, you will not need to do anything.

How the Test will Feel

How the test will feel depends on the method used to take a sample. You may not feel anything, or you may feel pressure and mild pain, such as during a biopsy. You may be given some form of pain medicine so you have little or no pain.

Why the Test is Performed

You may have this test to diagnose an infection caused by bacteria. It can also identify the type of bacteria causing the infection.

This test can help find the cause of various health problems, including:

  • Intestinal infection or illness
  • Sexually transmitted diseases (STDs)
  • Unexplained swelling or joint pain
  • Signs of a heart infection or fluid buildup in the thin sac that surrounds the heart (pericardium)
  • Signs of infection of the space around the lungs (pleural space)
  • Cough that will not go away, or if you are coughing up material with a foul odor or odd color
  • Infected skin sore

Normal Results

A normal result means that no bacteria or only friendly bacteria were found. Some types of bacteria normally live in certain areas of the body, such as the intestines. Bacteria normally don't live in other areas, such as the brain or spinal fluid.

Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

Abnormal results may indicate an infection. You may need further tests, such as a culture, to find out more about the infection.

Risks

Your risks depend on the method used to remove tissue or fluid from your body. You may have no risk at all. Other risks are rare, but may include:

  • Infection
  • Bleeding
  • Heart or lung puncture
  • Collapsed lung
  • Breathing problems
  • Scarring
Review Date: 4/18/2024

Reviewed By

Linda J. Vorvick, MD, Clinical Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

References

Plourde AR, Beavis KG. Specimen collection and handling for diagnosis of infectious diseases. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 66.

Wojewoda CM, Stempak LM. Medical bacteriology. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 57.

Disclaimer

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. No warranty of any kind, either expressed or implied, is made as to the accuracy, reliability, timeliness, or correctness of any translations made by a third-party service of the information provided herein into any other language. © 1997- A.D.A.M., a business unit of Ebix, Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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Gram stain - Illustration Thumbnail

Gram stain

A Gram stain is a test used to help identify bacteria. The tested sample can be taken from body fluids that do not normally contain bacteria, such as blood, urine, or cerebrospinal fluid. A sample can also be taken from the site of a suspected infection, such as the throat, lungs, genitals, or skin. Bacteria are classified as either Gram-positive or Gram-negative, based on how they color in reaction with the Gram stain. The Gram stain is colored purple. When combined with the bacteria in a sample, the stain will either stay purple inside the bacteria (Gram-positive), or it will turn pink (Gram-negative). Examples of Gram-positive bacteria include Streptococcus and Staphylococcus aureus, as well as bacteria that cause anthrax, diphtheria, and toxic shock syndrome. Examples of Gram-negative bacteria include Escherichia coli (E coli), Salmonella, Hemophilus influenzae, as well as many bacteria that cause urinary tract infections, pneumonia, or peritonitis. Gram stain can be done within a few hours. The results help providers choose the first antibiotics to use. Cultures of bacteria help identify specific bacteria, but take days to complete.

Illustration

 
 
Gram stain - Illustration Thumbnail

Gram stain

A Gram stain is a test used to help identify bacteria. The tested sample can be taken from body fluids that do not normally contain bacteria, such as blood, urine, or cerebrospinal fluid. A sample can also be taken from the site of a suspected infection, such as the throat, lungs, genitals, or skin. Bacteria are classified as either Gram-positive or Gram-negative, based on how they color in reaction with the Gram stain. The Gram stain is colored purple. When combined with the bacteria in a sample, the stain will either stay purple inside the bacteria (Gram-positive), or it will turn pink (Gram-negative). Examples of Gram-positive bacteria include Streptococcus and Staphylococcus aureus, as well as bacteria that cause anthrax, diphtheria, and toxic shock syndrome. Examples of Gram-negative bacteria include Escherichia coli (E coli), Salmonella, Hemophilus influenzae, as well as many bacteria that cause urinary tract infections, pneumonia, or peritonitis. Gram stain can be done within a few hours. The results help providers choose the first antibiotics to use. Cultures of bacteria help identify specific bacteria, but take days to complete.

Illustration

 
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