A sputum culture is used to test for pathogenic bacteria and sometimes fungi in the lower respiratory tract (bronchi and lungs). It is important to ensure that the sample is collected from the lower respiratory tract and not from saliva; otherwise the identified micro-organisms will not necessarily be those responsible for the reported respiratory symptoms.
The number of squamous cells (oral cavity cells) and polynuclear cells (white blood cells involved in the fight against infection) helps clarify the quality of the specimen. The report indicates whether the culture identifies only normal bacteria in the oral cavity (oropharyngeal flora) or the types of pathogenic bacteria present. For most identified strains, an antibiogram is added to determine whether this strain will be sensitive (S) or resistant (R) to different antibiotics available for treatment. Typically, only one type of bacteria will be responsible for the infection, but occasionally there will be two or more types of bacteria involved. If only normal oropharyngeal flora is present, the infection could be due to these normal bacteria, the pathogenic bacteria may not have been present in sufficient concentration in the sample collected, or the infection could be due to another cause. The sample must often be taken again, and multiple samples are sometimes ordered.