If your doctor suspects that you have a bacterial infection
A swab of cells or fluid taken from the site of infection
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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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?
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 a 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?
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?
Can I perform this test at home?
No. This test is performed by trained laboratory professionals.
On This Site
Tests: Susceptibility Testing, Bacterial Wound Culture, Blood Culture, CSF Analysis, Urine Culture, AFB Smear and Culture, Gonorrhoea, Stool Culture, Fungal Tests, Sputum Culture, Synovial Fluid Analysis, Pleural Fluid Analysis, Pericardial Fluid Analysis, Peritoneal Fluid Analysis
Conditions: Wound and Skin Infections, Staph Wound Infections, Meningitis and Encephalitis, Fungal Infections, PID, Septic Arthritis, Urinary Tract Infection, Pneumonia, Sepsis