Sputum Culture Testing and Results

Medically Reviewed by Jabeen Begum, MD on April 30, 2024
Written by Stacey Jones, MS, BA
7 min read

A sputum culture is a sample of the gooey substance that often comes up from your chest when you have an infection in your lungs or airways. It is mostly made up of white blood cells that fight infection mixed with germs.

Doctors use it to figure out what might be causing your illness, whether it’s bacteria, a virus, or something else.

What is sputum?

Your lungs create sputum, a type of mucus that can appear when you cough due to an infection or a chronic illness. You start to cough up sputum when your lungs can no longer get rid of it on their own.

Your lungs are connected to your mouth by a passage called the trachea, or windpipe, which starts at the back of your throat. A few inches down, it splits into separate channels called bronchi, which funnel air from the trachea into your lungs.

If you’re sick or the passages between your mouth and lungs get irritated by something such as smoke or air pollution, your body makes sputum. It’s also known as phlegm. It’s different from saliva, the thinner fluid your mouth makes to help you eat.

When you cough, your body is trying to get rid of that phlegm.

If your doctor isn’t sure what’s wrong with you, they might collect a sample of your sputum, or sputum culture, to test for illnesses.

Your doctor probably will ask you questions about your cough. Some of them might include:

  • How long has it been going on?
  • How long do your coughing spells last?
  • Does anything come up when you cough?
  • Is it worse during a certain time of day?
  • Do you smoke?
  • Have you lost weight?
  • Do you have night sweats?

Your answers to these and other questions will give your doctor some idea of what the problem is.

You might need to give a sputum culture if:

  • Your cough suggests you have an illness caused by bacteria, such as bronchitis, pneumonia or tuberculosis (a potentially serious infection that usually affects your lungs and can cause you to cough up blood).
  • You have a mild or serious lung infection with symptoms such as chest tightness, body aches, tiredness, shortness of breath, and wheezing.
  • Your cough might be caused by other microbes, such as a fungus or a virus. 

Common conditions such as colds, the flu, and viruses such as COVID-19 produce sputum when you cough. Producing excess sputum and triggering the urge to cough it up is your lungs' defense against disease or foreign substances. 

Viral tracheobronchitis, inflammation of your windpipe and bronchial passages, begins with a persistent but nonproductive cough. Later, your cough can produce a combination of mucus and pus, which means there's an infection. 

Long-term smoking can lead to a chronic cough – a cough lasting 3 or more weeks – typically the result of changes to the structure of your respiratory tract or ongoing irritants. This can turn into bronchitis, another infection that causes excessive sputum production.

Other conditions that increase your sputum production include:

  • Asthma or cystic fibrosis
  • Chronic postnatal drip
  • Lung cancer and chest tumors
  • Left-sided heart failure
  • Inhaling foreign particles
  • Lung abscesses
  • Fluid on the lungs

 

When you take a sputum culture test, you’ll be asked to try to cough up some sputum and spit it into a clean cup for testing. You might produce the sample at home or in your doctor's office. Usually, your doctor will have you test early in the morning when your sputum is more dense.

Your doctor will probably need about 1 teaspoon (5 milliliters) of sputum to run the test. Try to spit out as much sputum and as little saliva as you can. The test has no known risks.

How do I prepare for the sputum test?

You might need to rinse your mouth with water first, and your doctor might ask you to stop taking any bacteria-killing antibiotics or blood-thinning medicines you’ve been prescribed before the test.

Also:

  • Try to drink a lot of water the night before your test. This may make it easier for you to cough up a larger sample.
  • Don't use mouthwash before your test.
  • If you are having a bronchoscopy (a procedure that uses a scope to look at or biopsy your lungs), follow your doctor's instructions about when to stop eating or drinking (usually up to 12 hours) before your test.
  • If needed, arrange for someone to drive you home after your test.

.

A technician might be able to induce some sputum if you can’t do it on your own.

If you still can’t cough up enough sputum, your doctor might have you breathe in a mist of hypertonic (salty) water that causes a deeper cough to help expel sputum as well as rule out tuberculosis. 

In a more invasive procedure, they might have to use an instrument called a “bronchoscope” to collect a sample. The device has a light and a tiny camera. Your doctor gently inserts it down your windpipe to locate a sample. You’ll be given drugs to relax you while this happens, but you may be hoarse and have a sore throat afterward.

There is a small chance of bleeding, getting a fever or pneumonia, or having a collapsed lung during this process.

Your doctor will likely look at the color of what you’ve spat out. It can give clues as to what’s going on.

Once your doctor has looked at the sample, a lab technician can run tests that will show what kind of bacteria or cells it contains.

Those tests will help separate the normal bacteria that are in your body from the disease-causing bugs that may be making you sick. If an infection is found, other tests can clarify what antibiotic to prescribe.

It may take several days for a complete set of tests to be done. But if your sample suggests something dangerous, your doctor should be able to tell you right away.

Abnormal sputum culture test results

If your sputum culture is abnormal, the results are considered positive for the presence of a virus, fungus, or bacteria in your lungs; conditions such as pneumonia or tuberculosis; or a worsening of symptoms in diseases such as chronic obstructive pulmonary disease (COPD) or cystic fibrosis.

Normal sputum is clear, white, or sometimes gray. Other colors can help your doctor diagnose what's going on in your lungs.

  • Shades of yellow or green: Yellow sputum is probably a result of large amounts of infection-fighting white blood cells – a sign of a respiratory illness such as pneumonia or bronchitis or chronic inflammation.
  • Red or rusty: If you have a condition that causes bleeding, there might be streaks or spots of red in the sputum. Bloody or rust-colored sputum may point to irritation, infection, a disease such as cancer, or the presence of a blood clot.
  • Black: If you smoke, have worked in a sooty place like a coal mine, or have inhaled harmful air particles, your sputum might have a gray or black tinge to it.
  • Pink: This may signal heart failure.
  • Dark brown: This could mean an infection like bacterial pneumonia.

Depending on your results, your doctor might ask you to take other tests.

  • You might be asked to have an X-ray or CT scan to look for signs of an ongoing lung condition.
  • You might be given what’s called a “pulmonary function test” to find out how well your lungs are working.

A sputum test is given to find out you have bacteria, a virus, or other germs in your lungs or airways that may be causing discomfort or illness. Your doctor may also have you do regular sputum culture testing to monitor the health of your lungs after an illness or if you have a chronic lung condition. There are two ways to produce a sputum culture: by coughing it out of your lungs and into a sterile cup or with a procedure using a flexible tube inserted through your mouth called a bronchoscopy.

What is a sputum culture test for?

A sputum culture is used to see if there are bacteria or other germs that may cause lung infections or disease, for keeping an eye on certain lung conditions, or to see if your lung treatment is working.

What infections can be found in sputum?

A positive sputum culture may mean you have a lung infection such as pneumonia or tuberculosis.

What can be detected from a sputum sample?

A sputum culture can show if there's inflammation, bacteria, or a fungus in your lungs. Sputum cultures aren't usually given if a viral infection is suspected.

How long does it take to get a sputum culture back?

A culture is a lab test used to uncover germs that cause disease. In a sputum culture test, your sputum is added to a substance that promotes the growth of bacteria or other germs, if present. Because this process takes time, it usually takes several days to get the results of your sputum test back.