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Sputum Gram stain

Gram stain of sputum

A sputum Gram stain is a laboratory test used to detect bacteria in a sputum sample. Sputum is the material that comes up from your air passages when you cough very deeply.

The Gram stain method is one of the most commonly used methods to rapidly identify the cause of a bacterial infection, including pneumonia.

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Sputum test

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How the Test is Performed

A sputum sample is needed.

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. Stains are placed on 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 bacteria.

How to Prepare for the Test

Drinking fluids the night before the test helps your lungs produce phlegm. It makes the test more accurate if it is done first thing in the morning.

If you are having a bronchoscopy, follow your provider's instructions on how to prepare for the procedure.

How the Test will Feel

There is no discomfort, unless a bronchoscopy needs to be performed.

Why the Test is Performed

Your health care provider 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.

Normal Results

A normal result means that few to no white blood cells and no bacteria were seen in the sample. The sputum is clear, thin, and odorless.

What Abnormal Results Mean

An abnormal result means that bacteria are seen in the test sample. You may have a bacterial infection. A culture is needed to confirm the diagnosis.

Risks

There are no risks, unless bronchoscopy is performed.

References

Beavis KG, Charnot-Katsikas A. Specimen collection and handling for diagnosis of infectious diseases. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 64.

Torres A, Menendez R, Wunderink RG. Bacterial pneumonia and lung abscess. In: Broaddus VC, Mason RJ, Ernst JD, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 33.

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Review Date: 12/1/2018  

Reviewed By: Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School; Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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