Sputum can be obtained by spontaneous expectoration or induced by inhalation of an irritating aerosol like hypertonic saline. Sputum is distinguished from saliva by the presence of bronchial epithelial cells and alveolar macrophages as opposed to squamous epithelial cells. Sputum examination should include gross inspection for blood, color, and odor, as well as Gram's stain and routine bacterial culture. Bacterial culture of expectorated sputum may be misleading due to contamination with oropharyngeal flora. Sputum samples can also be assessed for a variety of other pathogens, including mycobacteria, fungi, and viruses. Sputum samples induced by hypertonic saline can be stained for the presence of Pneumocystis jirovecii. Cytologic examination of sputum samples can be used as an initial screen for malignancy.