What Is a Sputum Culture?

Medically Reviewed by Melinda Ratini, MS, DO on April 21, 2023
4 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.

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 to test for different illnesses.

Your doctor probably will ask you a series of 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.

But 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).
  • Your cough might be caused by other microbes, such as a fungus or a virus.

Most of the time, you’ll be asked to try to cough up some sputum and spit it into a clean cup for testing.

You might need to rinse your mouth with water first, and your doctor might ask you to skip a meal or stop taking any bacteria-killing antibiotics you’ve been given before the test.

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. There are no known risks to the 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 breath in a mist of hypertonic (salty) water that induces 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:

  • Off-white, yellow or green: This means your sputum probably includes large numbers of infection-fighting white blood cells -- a sign of a respiratory illness such as pneumonia or bronchitis.
  • 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 a more serious condition.
  • Gray or black: If you smoke or have worked in a sooty place like a coal mine, your sputum might have a gray or black tinge to it.

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 present in your body from the disease-causing bugs that may be making you sick. If an infection is found, additional 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.

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.