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?

Seek advice from your doctor if you have an area of inflamed, red or a painful skin, a wound which does not seem to be healing or any other concerns which might lead you to believe you may have a bacterial infection

Sample Required?

A skin swab or fluid/pus (if present) from the site of infection

Test Preparation Needed?

None

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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 after 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 pus containing cellular matter) at the site of suspected infection. Body fluids may be collected in sterile containers or by needle and syringe.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Common Questions

How is it used?

This is one of the most commonly performed microbiology tests used to identify the cause of an infection. Often, determining whether an infection is caused by an organism that is Gram positive or Gram negative will be sufficient to allow a doctor to prescribe treatment with an appropriate antibiotic while waiting for more specific tests to be completed. Absence or presence of white blood cells in the Gram stain can determine the adequacy of the specimen.
When is it requested?

A Gram stain is performed when a bacterial infection is suspected.
What does the test result mean?

Determining the Gram status of an organism allows the doctor to select an appropriate antibiotic before culture results are available. Gram stains cannot predict the organism’s identity or susceptibility to antibiotics. Only the culture of the material can supply this information.
Is there anything else I should know?

Bacterial infections should not be ignored, even if the symptoms are mild. You should consult your doctor so treatment, if necessary, can begin promptly, and the spread and severity of the disease can be limited. If left untreated, bacterial infections can migrate throughout the body and cause tissue and organ damage.
What are the treatment options for bacterial infections?

Antibiotics are the main treatment of bacterial infections.
What does the doctor look for on the slide?

In addition to colour, which indicates whether the organism is Gram positive or Gram negative, the shape of the organism (such as rod-shaped) and the formation of groups of organisms is informative. For example, staphylococcus (staph) bacteria are found in clusters while streptococcus (strep) bacteria are found in chains.
What happens if my doctor needs more information than the Gram stain provides?

The Gram stain is usually done in conjunction with a culture. This allows for more exact identification of the bacteria causing the infection and determination of the most appropriate antibiotic.
Can I perform this test at home?

No. This test must be performed in an accredited laboratory by trained laboratory professionals.

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