Sputum is mucus that is coughed up from the lower airways.[1] In medicine, sputum samples are usually used for microbiological investigations of respiratory infections and cytological investigation of respiratory systems. The best sputum samples contain very little saliva,[2] 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.[citation needed] 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.[citation needed] 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[3] contains pus, composed of white blood cells, cellular debris, dead tissue, serous fluid and viscous liquid (mucus). Mostly, it is yellow in color, as well as green. It is seen in cases of bronchiectasis, lung abscess, advanced stage of bronchitis, or acute upper respiratory tract infection (common cold, laryngitis).

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