A sputum culture is a test that checks for bacteria or another type of organism that may be causing an infection in your lungs or the airways leading to the lungs. Sputum, also known as phlegm, is a thick type of mucus made in your lungs. If you have an infection or chronic illness affecting the lungs or airways, it can make you cough up sputum.
Sputum is not the same as spit or saliva. Sputum contains cells from the immune system that help fight the bacteria, fungi, or other foreign substances in your lungs or airways. The thickness of sputum helps trap the foreign material. This allows cilia (tiny hairs) in the airways to push it through the mouth and be coughed out.