Sputum

Sputum is the mucus and secretions that are expectorated from the lower airways during coughing. It differs from saliva in origin and from phlegm in common usage; medically, sputum specifically refers to material produced by the bronchial tree that can be expectorated or coughed up. The specimen typically contains mucus with inflammatory cells, epithelial cells, microorganisms, and cellular debris, and its examination can aid in diagnosing respiratory infections and other lung conditions.

Sputum is collected by coughing up mucus into a sterile container. If a spontaneous sample cannot be obtained, sputum induction using inhaled hypertonic saline may be employed, under infection control precautions. Good-quality specimens are those not heavily contaminated by oropharyngeal secretions; the sample should largely originate from the lower respiratory tract. Laboratory assessment often uses a Gram stain and culture, and the specimen is evaluated for levels of epithelial cells and inflammatory cells to determine adequacy.

Laboratory testing includes direct microscopy with Gram stain to identify bacteria and cellular content, and microbial culture to determine the causative organism and guide antibiotic therapy. Special tests may be performed on sputum when indicated, such as acid-fast staining and molecular assays for Mycobacterium tuberculosis, fungal stains for certain pathogens, viral testing, or nucleic acid amplification tests. Sputum cytology can be used to detect malignant cells in suspected lung cancer. The results must be interpreted in the clinical context.

Color, viscosity, and the presence of blood are nonspecific cues and should be interpreted with caution. Sputum findings contribute to diagnosing pneumonia, bronchitis, bronchiectasis, tuberculosis, and other lung diseases, and are often combined with imaging and clinical assessment.