Sputum is a respiratory substance composed of saliva, pus and mucus, which is generally expelled out by mouth from the lower airways. The term originates from a Latin word sputus, which means to spit. This material is important in investigating microbiological infections of the respiratory system. Best specimens are those which contain only a little amount of saliva, as an increase would contaminate the sample with microorganisms. Thus, good samples are best obtained before treatment with antimicrobials via deep cough.

Sputum can be described in different ways, depending on its consistency, color and character. Bloody sputum is a blood streaked substance which could be a sign of bronchitis or lung cancer or may also be a massive haemoptysis that results from cavitary tuberculosis, bronchiectasis and infarction. A rusty colored blood suggests the presence of an infection which is pneumococcal in origin. Sputum that contains WBC, cellular debris, dead cells, serous fluid and mucus is called purulent sputum. Purulent sputum may appear as a yellowish to greenish mucopurulent substance which is suggestive of an advanced bronchitis, URTI and lung abscess. Purulent sputum can also be milky white in appearance, and this indicates a viral infection, thus antibiotics will surely be unsuccessful in treating the symptoms. Other descriptions include a foamy white sputum and frothy pink sputum which are present when there is fluid accumulation in the lungs.