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Tests & Procedures : Lab Tests

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Sputum Culture

Does this test have other names?

No.

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, or COPD, can also cause coughing and make it difficult to breathe. This test helps your doctor 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 doctor decide on the right treatment for you.

Why do I need this test?

You may have this test if your doctor 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 doctor may also order an X-ray, blood culture, sputum Gram stain, or urine antigen test to find the source of your infection.

If your doctor 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 health care 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 doctor 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 doctor 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 one to two hours before the test, or longer if your doctor plans to use a bronchoscope to get the sample.