Gram Stain

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Also known as: Gram's Stain
Formal name: Gram Stain

At a Glance

Why Get Tested?

To identify the cause of a bacterial infection so appropriate treatment can be given

When to Get Tested?

If your doctor suspects that you have a bacterial infection

Sample Required?

A swab of cells or fluid taken from the site of infection

The Test Sample

What is being tested?

A Gram stain is used to determine if bacteria are present in an area of the body that is normally sterile, such as spinal fluid. A sample from the infected area is smeared on a glass slide and allowed to dry. A series of stains are applied and then the stained slide is examined under a microscope where bacteria appear either purple (gram positive) or pink (gram negative). The test is named for Dr. Christian Gram, who invented the process.

A Gram stain can predict the type of bacteria causing an infection, such as pneumococcal pneumonia or a staphylococcal abscess. Viruses cannot be seen with a Gram stain since they lack the cell wall, which takes up the stain.

How is the sample collected for testing?

Usually, samples are collected using sterile swabs to obtain cells or exudate (fluid or ooze containing cellular matter) at the site of suspected infection. Body fluids may be collected in sterile containers or by needle and syringe.

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.

The Test

Common Questions

Ask a Laboratory Scientist

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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.