Lower respiratory tract infections (LRTI) are infections occurring below the level of the larynx, i.e., in the trachea, the bronchi, or in the lung tissue (tracheitis, bronchitis, lung abscess, pneumonia). Sometimes, in pneumonia, the adjacent membranous covering of the lung is involved, resulting in roughening (pleurisy) and sometimes production of fluid in the pleural cavity (pleural effusion).
A special form of LRTI is pulmonary tuberculosis, which is common in many countries. The patient may cough up aerosols containing tubercle bacilli (Mycobacterium tuberculosis) which can be inhaled by other people. This form of the disease (“open” tuberculosis) is easily spread from person to person, and is therefore a serious communicable disease.
Many patients with LRTI cough up purulent (pus-containing) sputum that is generally green or yellowish in colour; this sputum may be cultured and examined grossly and microscopically.
There are other infections in which little or no sputum is produced: Legionnaires’ disease (caused by Legionella pneumophila) and pneumonia due to Mycoplasma pneumoniae (“primary atypical pneumonia”). Both these diseases require specialized techniques for their diagnosis and will not be discussed further here. Apart from pulmonary tuberculosis (see below), most requests for sputum microscopy and culture concern patients with respiratory infections associated with purulent sputum.