Sputum Culture

Sputum cultures detect the presence of pathogenic bacteria in those who have bacterial pneumonia and other lower respiratory tract infections. Bacteria in the sample are identified and susceptibility testingis performed to guide antimicrobial treatment.

Sputum is the thick mucus or phlegm that is expelled from the lower respiratory tract (bronchi and lungs) through coughing; it is not saliva or spit. Care must be taken in the sample collection process to ensure that the sample is from the lower airways and not the upper respiratory tract. If a sample is mostly saliva, the microorganisms grown in culture will not be representative of those causing the infection. Furthermore, the presence of saliva and bacteria from the mouth in a sputum sample make it more difficult to identify pathogenic bacteria in the lungs.

The first step in the analysis of a fresh sputum sample is a Gram stain test to identify the general type of bacteria that may be present and to determine sample adequacy. If a sample contains a significant number of normal cells that line the mouth (squamous epithelial cells), then the sample is not generally considered adequate for culture and a re-collection of the sample may be required. If the sample contains a majority of white blood cells that indicate a host response to an infection, then it is considered to be an adequate sample for culturing.

Once a sputum sample has been accepted, it is placed on or in appropriate nutrient media and incubated. The media encourages the growth of bacteria that are present, allowing for further testing and identification. Sputum is not sterile, so when a person has a bacterial respiratory infection, there will typically be both normal flora and any pathogens present.

The next step is to identify the different types of bacteria present and categorize them as normal flora or a potential pathogen causing an infection. Identification is a step-by-step process that may involve several biochemical tests and observations of the organism's growth characteristics.

Antimicrobial susceptibility testing is frequently required to guide the treatment of identified pathogens and to determine whether the bacteria present are likely to respond to specific antibiotics.

The sputum culture, Gram stain(s), and susceptibility testing all contribute to a report that informs the doctor which pathogen(s) are present and what antibiotic therapy is likely to inhibit their growth.

The infection may be caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture, so that other tests, such as an AFB smear and culture, fungal culture, or viral culture, may be ordered in addition to or instead of a routine culture.

Sputum samples may be expectorated or induced. Expectorated samples are coughed up and expelled into a sterile cup provided by the laboratory. The person's mouth should be rinsed with water or saline prior to sample collection. Deep coughing is generally required, and the person should be informed that it is phlegm/mucus from the lungs that is necessary, not saliva. If someone cannot produce a sputum sample, then it can often be induced by inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs.

All samples collected should be taken to the laboratory promptly for processing while they are fresh. Sputum samples must be evaluated and accepted by the laboratory before they are processed. Useful sputum culture results rely heavily on good sample collection. A sample that is not considered "adequate" must be rejected and a recollection requested. Adequate means that it is considered representative of conditions in the lower respiratory tract.

Rinse mouth out with water prior to collection to remove loose cells in the mouth.