However, blood-tinged or bloodstained sputum is considered acceptable. The presence of food debris and contaminants is not advised. It is important to differentiate salivary sputum specimens from saliva and mucus, as the latter are not representative of lung status and may give false-negative results. Suboptimal samples include mucoid, mucosalivary, and salivary specimens. Purulent and mucopurulent sputum are considered good specimens for microscopy.
Sputum collection must be performed in dedicated areas that are well ventilated to prevent the nosocomial inhalation of aerosols by uninfected people. To yield a better sample, gastric aspiration should be done early morning, when the patient has not had anything to eat. This procedure is particularly useful in children. Gastric aspiration can be used to obtain sputum previously expectorated and swallowed by the patient.