Test Details: Sputum, also known as phlegm, is a thick mucus made in lungs. If an individual has an infection or chronic illness affecting the lungs or airways, it can make them cough up sputum. Yellowish-green sputum is a sign of lung infection (like Pneumonia). A sputum culture is a test that checks for bacteria or another type of organism that may be causing an upper respiratory tract infection. It also checks if a chronic illness of the lungs has worsened and to see if treatment for an infection is working.