Sputum is matter that is expelled from the respiratory tract, such as mucus or phlegm, mixed with saliva, which can then be spat from the mouth. It is usually associated with air passages in diseased lungs, bronchi, or upper respiratory tract. It can be found to contain blood if in a chronic cough possibly from severe cases of tuberculosis.

A sputum sample is the name given to the mucus that is coughed up from the lower airways. It is usually used for microbiological investigations of respiratory infections.

The best sputum samples contain very little saliva, as this contaminates the sample with oral bacteria. This event is assessed by the clinical microbiologist by examining a Gram stain of the sputum. More than 25 squamous epithelial cells at low enlargement indicates salivary contamination.

When a sputum specimen is plated out, it is best to get the portion of the sample that most looks like pus onto the swab. If there is any blood in the sputum, this should also be on the swab.

Microbiological sputum samples are usually used to look for infections by Moraxella catarrhalis, Mycobacterium tuberculosis, Streptococcus pneumoniae and Haemophilus influenzae. Other pathogens can also be found.

Purulent Sputum is that containing, or consisting of, pus.

Sputum can be:

1. Bloody (often found in tuberculosis) (Haemoptysis)
2. Rusty colored - usually caused by pneumococcal bacteria (in pneumonia)
3. Purulent - containing pus. The color gives hints about probable causes:
   1. a yellow-greenish (mucopurulent) color points to a bacterial infection.
   2. a white, milky, or opaque (mucoid) appearance often means a viral infection.
4. Foamy white - may come from obstruction or even Edema