Sputum Culture

By Pascual, Psyche 
March 22, 2017

Sputum Culture

Does this test have other names?


What is this test?

This test finds out what's causing your lung infection. Sputum, or phlegm, is the mucus that settles in the lower airways of your lungs when you have an infection or a chronic illness.

A lung infection like pneumonia can cause you to cough up phlegm. Other conditions, including bronchiectasis and chronic obstructive pulmonary disease (COPD), can also cause coughing and make it difficult to breathe. This test helps your healthcare provider find out whether the buildup of phlegm in your lungs is caused by bacteria, fungi, or another microorganism. Knowing the cause of your infection can help your provider decide on the right treatment for you.

Why do I need this test?

You may have this test if your healthcare provider suspects that you have a lung infection, such as pneumonia, especially if you're coughing up a lot of phlegm.

You may also have this test if you have severe pneumonia and are admitted to the hospital.

What other tests might I have along with this test?

Your healthcare provider may also order an X-ray, blood culture, sputum Gram stain, or urine antigen test to find the source of your infection.

If your provider suspects a chronic lung condition, you may have other tests, including:

  • CT scan

  • Pulmonary function test to see how well your lungs work

  • Complete blood count

  • Erythrocyte sedimentation rate and C-reactive protein tests to look for inflammation

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

Normal results are negative, meaning you don't have a lung infection. A positive result means bacteria were found, but it may not pinpoint the exact cause of your condition. Some bacteria are normally found in the lungs and cause no problems. Your healthcare provider may have to do other tests to a make a diagnosis.

How is this test done?

The test requires a sputum sample from deep inside your lungs. You may need to rinse your mouth with water first, then cough up enough mucus for a culture sample to be taken. If you can't cough up enough fluid, your healthcare provider may use a bronchoscope to get a sample for testing. This instrument is a narrow, flexible tube with a light on it.

Does this test pose any risks?

This test poses no known risks.

What might affect my test results?

Taking antibiotics can cause a false-negative result, because these medicines may kill the bacteria causing the problem.

How do I prepare for the test?

You may need to stop taking antibiotics before the test. You may also need to avoid eating for 1 to 2 hours before the test, or longer if your healthcare provider plans to use a bronchoscope to get the sample.



March 22, 2017


Bronchiecstasis. Murray and Nadel's Textbook of Respiratory Medicine. Mason RJ. 2010, 5th ed., Sputum cultures fo the evaluation of bacterial pneumonia. UpToDate.

Reviewed By:  

Turley, Ray, BSN, MSN,Walton-Ziegler, Olivia, MS, PA-C