At a Glance
Why Get Tested?
To identify the presence and general type of microorganisms in a sample taken from the site of suspected infection or a sample of microorganism grown in culture so that further identification tests can be performed and appropriate treatment can be given
When to Get Tested?
When your doctor suspects that you have a bacterial infection; often whenever a culture is requested
Test Preparation Needed?
The Test Sample
What is being tested?
A Gram stain is a laboratory procedure used to detect the presence of microorganisms, especially bacteria, in a sample taken from the site of a suspected infection. It gives relatively quick results as to the general type of bacteria that may be present. The Gram stain involves applying a sample from the infected area onto a glass slide and allowing it to dry. The slide is then treated with a special stain and examined under a microscope by a trained laboratorian. Any bacteria that may be present are categorized by color and shape during the microscopic evaluation:
- Color — typically bacteria may be either "Gram positive" (purple) or "Gram negative" (pink)
- Shape — the most common shapes include round (cocci) or rod-shaped (bacilli)
Additional information may be obtained by observing the groupings of the bacteria on the slide, such as cocci that are present singly, in pairs, in groups of four, in clusters or in chains, or bacilli that are thick, thin, short, long, or have enlarged spores on one end. Any bacteria that are present within the patient's white blood cell (intracellular) are also noted. The Gram stain color and the bacterial shape give clues as to what microorganism might be causing the infection. Examples of gram-positive cocci include Staphylococcus aureus, the bacterium associated with staph infections. An example of gram-negative bacterium is Escherichia coli, the cause of many urinary tract infections. Fungi (in the form of yeasts or molds) can also be initially identified with the Gram stain, but viruses cannot be seen with a Gram stain.
How is the sample collected for testing?
Several different types of samples may be collected for Gram stains. Some samples are collected using sterile swabs to obtain cells or exudate at the site of suspected infection. Other samples, such as urine or sputum, may be collected in a sterile container. Some body fluids may be collected by needle and syringe. A swab may be used to collect a sample of bacteria grown and isolated in a culture.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
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Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1564-1565.
Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 80-83.
Vorvick, L. (Updated 2009 August 9). Endocervical gram stain. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003753.htm. Accessed February 2010.
Yuki Uehara, Y. et. al. (2009 September 18). Impact of Reporting Gram Stain Results from Blood Culture Bottles on the Selection of Antimicrobial Agents. Medscape Today from American Journal of Clinical Pathology [On-line information]. Available online at http://www.medscape.com/viewarticle/708594 through http://www.medscape.com. Accessed February 2010.
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pp 1016-1017.