A test called a sputum culture looks for bacteria or any other kind of organism that might be infecting your lungs or the airways leading to your lungs. Sputum sometimes referred to as phlegm is a viscous mucus produced in your lungs. You may cough up sputum if you have an infection or a longterm condition that affects your lungs or airways. Sputum is made up of immune system cells that support your body's defences against bacteria fungi and other foreign invaders in your lungs and airways. Sputum's thickness aids in capturing the foreign object. It can then be coughed out thanks to the cilia (tiny hairs) in the airways which push it out the mouth.