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Sputum evaluation (and sputum induction)

Medically reviewed by Last updated on Mar 24, 2023.

What is the test?

Harvard Health Publishing

If your doctor thinks you have pneumonia, he or she might examine a sample of your sputum, the phlegm that you cough out of your lungs, to try to determine what type of bacteria or other infectious agent might be the cause.

How do I prepare for the test?

Drink plenty of fluids the night before the test; this may help to produce a sample.

What happens when the test is performed?

You need to cough up a sample of sputum. To be useful for testing, the stuff you cough up has to be from deep within the lungs. If your cough is too shallow or dry, the doctor might ask you to breathe in a saltwater mist through a tube or mask. This mist makes you cough deeply that helps produce an excellent phlegm sample.

The sputum sample is collected in a sterile container. Your doctor may remove a couple drops to put on a slide. The slide is stained with a special solution that colors the bacteria and white blood cells. The doctor may look at the slide under the microscope. More often the sample is sent to the laboratory and technician does the staining and microscopic examination.

The rest of the sputum sample is incubated to grow the bacteria or other germs in it for further testing. This step is called a sputum culture.

What risks are there from the test?


Must I do anything special after the test is over?


How long is it before the result of the test is known?

While some stain results might be available on the day of your test, the culture usually requires several days.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.