Gram stain of sputum

Alternative names
Sputum Gram stain


Gram stain is a method of staining microorganisms (bacteria) using a special series of stains. In this test, a sputum specimen is examined under the microscope. Sputum is the mucous secretions produced by the lower respiratory tract (in the lungs).

The Gram stain method can be applied to almost any clinical specimen, and is one of the most commonly used techniques for the rapid diagnosis of bacterial infections.

How the test is performed

A sample of sputum is applied in a very thin layer to a microscope slide - this is called a smear. A series of stains called a Gram stain is applied to the specimen. It is first stained with crystal violet stain, then iodine, then decolorized, then stained with safranin.

The stained smear is then examined under the microscope for the presence of bacteria. The color, size, and morphologic appearance (shape) of the cells help identify the infecting organism.

How to prepare for the test

You will produce a sputum sample from your lungs by coughing a specimen of mucus (not saliva or spit from the mouth) from deep inside your lungs into a container. If coughing does not produce sputum, a breathing treatment may precede the test to induce or encourage sputum production.

If you have a dry cough or are unable to produce a specimen, bronchoscopy may be necessary.

How the test will feel
The person needing the test will usually have a productive cough. Occasionally, the sputum is collected by bronchoscopy, and more discomfort is associated with this procedure.

Why the test is performed
The test is performed when there is a persistent or prolonged cough, sputum has a foul odor or unusual color, respiratory disease is suspected, or there is a generalized infection.

Normal Values
No presence of organisms is normal. The sputum is clear, thin, and odorless.

What abnormal results mean
A bacterial infection may be present. The infecting organisms may be tentatively identified. Culture is needed to confirm the diagnosis.

What the risks are
There are no risks unless a bronchoscopy is used to obtain the specimen.

Special considerations
The test may need to be repeated if the specimen produced contains only saliva from the mouth.

Johns Hopkins patient information

Last revised: December 7, 2012
by Mamikon Bozoyan, M.D.

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