A sputum Gram stain is a laboratory test that uses a series of stains to check for bacteria in a sputum sample. Sputum is the mucous material that comes up from your air passages when you cough very deeply.

The Gram stain method is one of the most commonly used techniques for the rapid diagnosis of bacterial infections, including bacterial pneumonia.

Your doctor may order this test if you have a persistent or prolonged cough, or if you are coughing up material that has a foul odor or unusual color. The test may also be done if you have other signs and symptoms of respiratory disease or infection.

A sputum sample is needed. You will be asked to cough deeply and spit any material that comes up from your lung into a special container. If this does not work, you may receive a breathing treatment before the test to help you produce the sample. If you have a dry cough or are unable to produce a sample, a bronchoscopy may be necessary.

The sample is sent to a lab. The lab team member places a very thin layer of the sample onto a glass slide -- this is called a smear. A series of special stains is applied to the sample. The lab team member looks at the stained slide under a microscope, checking for bacteria and white blood cells. The color, size, and shape of the cells help identify the infecting organism.

No preparation is usually needed, unless a bronchoscopy is necessary.

How the test will feel depends on the method used to produce the sample. More discomfort is associated with a bronchoscopy.