sputum

Sputum is mucus that has been coughed up from the lower airways and reaches the mouth, where it can be seen and, importantly, collected. It is the lungs' way of handing over a sample of what is going on deep inside — a readable message brought up from below.

Clinically, sputum is distinguished from saliva, which comes only from the mouth: true sputum originates in the trachea and bronchi and is brought up by a deep cough. Its appearance gives clues — clear and watery, thick and yellow-green with infection, frothy and pink in pulmonary edema, or rust-colored in classic pneumococcal pneumonia.

Because it carries cells, microbes, and chemicals from the airways, sputum is a valuable, non-invasive diagnostic specimen. It is examined under the microscope, cultured to identify infecting bacteria, stained for the tuberculosis bacillus, tested with molecular methods, and sometimes checked for cancer cells. Getting a good sample sometimes needs help, such as inhaling salt mist to induce a productive cough.