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Sputum Culture

Test Overview

A sputum culture is a test to detect and identify bacteria or fungi (plural of fungus) that are infecting the lungs or breathing passages. Sputum is a thick fluid produced in the lungs and in the airways leading to the lungs. A sample of sputum is placed in a container with substances that promote the growth of bacteria or fungi. If no bacteria or fungi grow, the culture is negative. If organisms that can cause infection grow, the culture is positive. The type of bacterium or fungus will be identified with a microscope or by chemical tests.

If bacteria or fungi that can cause infection grow in the culture, other tests may be done to determine which antibiotic will be most effective in treating the infection. This is called susceptibility or sensitivity testing.

This test is done on a sample of sputum that is usually collected by coughing. For people who can't cough deeply enough to produce a sample, they can breathe in a mist solution to help them cough.

Why It Is Done

A sputum culture is done to:

  • Find and identify bacteria or fungi that are causing an infection (such as pneumonia or tuberculosis) of the lungs or the airways leading to the lungs. Symptoms of a lung infection may include difficulty breathing, pain when breathing, or a cough that produces bloody or greenish brown sputum.
  • Identify the best antibiotic to treat the infection (sensitivity testing).
  • Monitor treatment of an infection.

How To Prepare

Do not use mouthwash before collecting a sputum sample because it may contain antibacterial substances that could affect your results.

If bronchoscopy will be used to collect your sputum sample, your doctor will tell you how soon before the test to stop eating and drinking. Follow the instructions exactly about when to stop eating and drinking, or your surgery may be canceled. If your doctor has instructed you to take your medicines on the day of surgery, please do so using only a sip of water.

Tell your doctor if you have recently taken any antibiotics.

How It Is Done

Usually, the sputum sample is collected early in the morning before you eat or drink anything. In some cases (especially if you may have tuberculosis), three or more morning samples may be needed.

If you wear dentures, you will need to remove them before collecting a sputum sample. Then rinse your mouth with water, take a deep breath, and then cough deeply to produce a sample of sputum. The health professional collecting the sample may tap on your chest to help loosen the sputum in your lungs before you cough. If you still have trouble coughing up a sample, you may be asked to inhale an aerosol mist to help you cough.

Bronchoscopy

Some people may need bronchoscopy to collect a sputum sample. During bronchoscopy, a thin, lighted tube (bronchoscope) is inserted through your mouth or nose into the airways leading to your lungs. You will be given medicine that numbs your throat and nose so you do not feel discomfort from the bronchoscope. You may also be given a sedative to make you sleepy during the procedure. To collect the sputum sample, a salt solution may be washed into the airway and then suctioned into a container. A small, thin brush may be used to collect a sample.

Suction

A sputum sample can also be collected using suction. During this procedure, a soft, flexible tube (called a nasotracheal catheter) is inserted through the nose and down the throat. Suction is applied for up to 15 seconds to collect the sputum sample. This method of collecting a sputum sample is often used for people who are very sick or unconscious.

After a sample is collected

Once the sputum sample is collected, it will be placed in a container with substances (growth medium or culture medium) that promote the growth of infecting organisms (bacteria or fungi). Bacteria usually need 2 to 3 days to grow. Fungus often takes a week or longer to grow. The organism that causes tuberculosis may take 6 weeks to grow. Any bacteria or fungi that grow will be identified under a microscope or by chemical tests. Sensitivity testing, to determine the best antibiotic to use against the organism that grows, often takes 1 to 2 more days.

How It Feels

If you have discomfort when taking a deep breath or when coughing, obtaining a sputum sample may be uncomfortable. If you need to inhale the aerosol mist to produce a sputum sample, you will often feel a deep, uncontrollable urge to cough.

During bronchoscopy or collection of a sputum sample using a catheter, you may feel a strong urge to cough as the bronchoscope or catheter passes into the back of your throat. You may also feel as if you cannot breathe. Try to relax and breathe slowly while the bronchoscope or catheter is in place. If you are given medicine to numb your throat and nose, you may feel as if your tongue and throat are swollen and that you cannot swallow.

Risks

Your throat may feel sore following bronchoscopy or collection of a sputum sample using a nasotracheal catheter.

If you have severe asthma or bronchitis, you may find it hard to breathe during collection of a sputum sample using a nasotracheal catheter.

Results

A sputum culture is a test to detect and identify bacteria or fungi that are infecting the lungs or breathing passages. Some types of bacteria or fungi grow quickly in a culture and some grow slowly. Test results may take from one day to several weeks, depending on the type of infection suspected. Some organisms (such as Chlamydophila pneumoniae and mycoplasma) do not grow in a standard culture and need a special growth medium to be detected in a sputum culture.

Sputum culture
Normal:

Sputum that has passed through the mouth normally contains several types of harmless bacteria, including some types of strep (Streptococcus) and staph (Staphylococcus). The culture should not show any harmful bacteria or fungi. Normal culture results are negative.

Abnormal:

Harmful bacteria or fungi are present. The most common harmful bacteria in a sputum culture are those that can cause bronchitis, pneumonia, or tuberculosis. If harmful bacteria or fungi grow, the culture is positive.

If test results point to an infection, sensitivity testing may be done to determine the best antibiotic to kill the bacteria or fungus.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Recent use of antibiotics, which may prevent the growth of bacteria or fungi in the culture.
  • Contamination of the sputum sample.
  • An inadequate sputum sample.
  • Waiting too long to deliver the sample to the laboratory.
  • Use of mouthwash before collecting a sputum sample.

What To Think About

  • Organisms (such as bacteria or a fungus) may be identified by using special dyes on the sputum sample. This is called a Gram stain. A Gram stain can help a doctor:
    • Determine whether the sputum sample is adequate for culture. For example, a sputum sample that is not collected properly may not contain enough bacteria to warrant a culture, or the sample may be contaminated by other bacteria that are not infecting the lungs. If the sample is not adequate for culture, another sample can be obtained.
    • Make an initial diagnosis before the culture results are received. In some cases, Gram stain results can be available within 30 minutes, but culture results may not be available for one to several days. Information received from a Gram stain can help your doctor treat the infection earlier, rather than waiting for the culture results.
  • Because sputum collected by coughing usually contains bacteria from your mouth, these culture results must be considered along with your symptoms, condition, and other test results, such as a chest X-ray.
  • A culture that does not grow any bacteria or fungi may not rule out an infection.
  • A special growth medium is needed for some cultures, such as for those that can find infection by tuberculosis (TB) bacteria, mycoplasma, Chlamydophila pneumoniae, or fungus. TB bacteria and most types of fungi grow very slowly and may not show up in a culture for several weeks. So treatment for a suspected fungus or TB infection may begin before culture results are known.
  • Sensitivity testing helps a doctor choose the best medicine to kill specific types of bacteria or fungi infecting a person. Differences in the genetic material (DNA) of bacteria or fungi may make them resistant to certain antibiotics. In such cases, those antibiotics can't kill all of the bacteria. When an effective antibiotic is chosen, you must complete the entire course to lower the chance that the bacteria will become resistant to the antibiotic. Stopping antibiotic treatment early kills only the most sensitive bacteria, while those that are more resistant can multiply and prolong the infection. Subsequent infections may also be harder to treat if caused by resistant bacteria.
  • Bronchoscopy is often done if a serious or recurring lung infection is present, especially when other tests have not definitely determined the cause. To learn more, see the topic Bronchoscopy.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.

Credits

By Healthwise Staff
Adam Husney, MD - Family Medicine
Robert L. Cowie, MB, FCP(SA), MD, MSc, MFOM - Pulmonology
Last Revised November 1, 2012

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