Sputum is mucus that is coughed up from the lower airways (the trachea and bronchi). In medicine, sputum samples are usually used for a naked eye examination, microbiological investigation of respiratory infections and cytological investigations of respiratory systems. It is crucial that the specimen does not include any mucoid material from the nose or oral cavity.

A naked eye exam of the sputum can be done at home by a patient in order to note the various colors (see below). Any hint of yellow or green color (pus) suggests an airway infection (but does not indicate the type of organism causing it). Such color hints are best detected when the sputum is viewed on a very white background such as white paper, a white pot or a white sink surface. The more intense the yellow color, the more likely it is a bacterial infection (bronchitis, bronchopneumonia or pneumonia).

Having green, yellow, or thickened phlegm (sputum) does not always indicate the presence of an infection. Also, if an infection is present, the color of the phlegm (sputum) does not determine whether a virus, a bacterium or another pathogen has caused it. Simple allergies can also cause changes in the color of the mucus.