Sputum is a thick fluid made in the lungs and in the airways leading to the lungs. A sputum culture is a test to find germs (such as bacteria or a fungus) that can cause an infection. A sample of sputum is added to a substance that promotes the growth of germs. If no germs grow, the culture is negative. If germs that can cause infection grow, the culture is positive. The type of germ may be identified using a microscope or chemical tests. Sometimes other tests are done to find the right medicine for treating the infection. This is called sensitivity testing.
You may be asked to cough to provide the sputum sample. Some people can't cough deeply enough to produce a sample. They can breathe in a special mist to help them cough.
Why It Is Done
A sputum culture is done to:
- Find bacteria or fungi that are causing an infection of the lungs or the airways leading to the lungs. Examples include pneumonia and tuberculosis. Symptoms of a lung infection may include having a hard time breathing, pain when breathing, or a cough that produces bloody or greenish brown sputum.
- Find the best antibiotic to treat the infection. (This is called sensitivity testing.)
- See if treatment is working.
How To Prepare
Do not use mouthwash before you collect your sputum sample. Some types of mouthwash can kill bacteria and 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. If you don't, your surgery may be cancelled. If your doctor told you to take your medicines on the day of surgery, take them with only a sip of water.
Tell your doctor if you have recently taken antibiotics.
How It Is Done
In most cases, the sputum sample is collected early in the morning before you eat or drink anything. In some cases, three or more morning samples may be needed. (This is often done if you may have tuberculosis.)
If you wear dentures, you will need to take them out before you collect your sputum sample. Then rinse your mouth with water. Next, take a deep breath and cough deeply to get a sample of sputum. The person taking the sample may tap on your chest. This tapping helps 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.
Some people may need bronchoscopy to collect a sputum sample. A thin, lighted tube (bronchoscope) is put through your mouth or nose into the airways leading to your lungs. You will get medicine that numbs your throat and nose so you do not feel pain from the bronchoscope. You may also get a sedative to make you sleepy during the test. To collect the 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 can also be used to collect a sputum sample. A soft, flexible tube (called a nasotracheal catheter) is put through the nose and down the throat. Suction is applied for up to 15 seconds to collect the sample. This method is often used for people who are very sick or unconscious.
After a sample is collected
The sputum sample will be placed in a container with a growth medium or culture medium. These are substances that help with the growth of 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 found under a microscope or by chemical tests. Sensitivity testing, to find the best antibiotic to use, often takes 1 to 2 more days.
How It Feels
If you have some pain when you take a deep breath or when you cough, giving a sputum sample may be uncomfortable. If you need to inhale the aerosol mist to produce a sample, you will likely feel a strong urge to cough.
During bronchoscopy or collection of a sputum sample using a catheter, you may feel a strong urge to cough. This can happen as the bronchoscope or catheter passes into the back of your throat. You may also feel as if you can't breathe. Try to relax. Breathe slowly.
If you are given medicine to numb your throat and nose, you may feel as if your tongue and throat are swollen. You may find it hard to swallow.
Your throat may feel sore after 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.
A sputum culture is a test to find bacteria or fungi that are infecting the lungs or airways. Some types of bacteria or fungi grow quickly in a culture, and some grow slowly. Test results may take from 1 day to several weeks. How long your results take depends on the type of infection your doctor thinks you may have. Some organisms do not grow in a standard culture and need a special growth medium to be found in a sputum culture. (Examples are Chlamydophila pneumoniae and mycoplasma.)
Sputum that has passed through the mouth normally contains several types of harmless bacteria. These include some types of strep ( Streptococcus) and staph ( Staphylococcus). The culture should not show any harmful bacteria or fungi. Normal culture results are negative.
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. This testing helps to find the best antibiotic to kill the bacteria or fungus.
Even if your culture does not grow any bacteria or fungi, you may still have an infection.
What Affects the Test
You may not be able to have the test, or the results may not be helpful, if:
- You used antibiotics recently. They may prevent the growth of bacteria or fungi in the culture.
- The sputum sample is contaminated.
- There is not enough sputum sample.
- You wait too long to take the sample to the lab.
- You use mouthwash before you collect your sputum sample.
What To Think About
- Bacteria or a fungus may be found by using special dyes on the sputum sample. This is called a Gram stain. A Gram stain can help a doctor:
- Find out if the sputum sample is good enough for culture. For example, was the sputum collected in the right way? Is there enough sputum in the sample? Does it contain other bacteria that aren't infecting the lungs? If the sample is not good enough for culture, another sample can be obtained.
- Make a diagnosis before the culture results come back from the lab. In some cases, Gram stain results can be available within 30 minutes. But other culture results may not be available for 1 to several days. Information received from a Gram stain can help your doctor treat the infection sooner, rather than waiting for the culture results.
- Sputum obtained by coughing most often contains bacteria from your mouth. So the doctor will consider your symptoms, condition, and other test results (such as a chest X-ray) along with your culture results.
- Sensitivity testing helps a doctor choose the best medicine to kill certain 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 antibiotic is chosen, you must take all of the medicine to lower the chance that the bacteria will become resistant to the antibiotic. Stopping antibiotic treatment early kills only the weakest bacteria. Then those that are stronger and more resistant can multiply and prolong the infection. Later infections may also be harder to treat if they are caused by resistant bacteria.
- Bronchoscopy is often done if a serious or recurring lung infection is present. This is especially true when other tests have not definitely found the cause. To learn more, see the topic Bronchoscopy.
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.
Current as of: September 23, 2020
Author: Healthwise Staff
Adam Husney MD - Family Medicine
E. Gregory Thompson MD - Internal Medicine
Kathleen Romito MD - Family Medicine
W. David Colby IV MSc, MD, FRCPC - Infectious Disease