Sputum Culture

Sputum cultures detect the presence of bacteria which have the potential to cause infection. These bacteria in the sample are identified and their susceptibility to various antibiotics is assessed to help choose the most appropriate antibiotic (antimicrobial) treatment.

Sputum is the thick mucus or phlegm that is coughed up from the lower respiratory tract; it is not saliva or spit. If a sample is mostly saliva, the microorganisms grown in culture will not necessarily be those causing the infection. Furthermore, the presence of saliva and bacteria from the mouth in a sputum sample make it more difficult to identify disease-causing bacteria in the lungs.

Sputum is broken up with a substance called mucolyse. This ensures that the bacteria present in the sample is evenly distributed throughout the sample. Once a sputum sample is fully homogenised it is placed on appropriate nutrient media and incubated under conditions which simulate body temperature. The media encourages bacteria to grow which allows further testing and identification. Sputum is not normally free of bacteria, so when a person has a bacterial respiratory infection, there will typically be both normal and infection-causing bacteria present.

The next step is to identify the different types of bacteria present and categorise them as normal or potential infection-causing bacteria. Differentiation is performed by a trained scientist and suspected infection causing bacteria are followed up with confirmatory tests.

'Antimicrobial susceptibility testing’ is frequently used to find out whether they are likely to respond to particular antibiotics and determine the best treatment option.

The sputum culture and susceptibility testing all contribute to helping the doctor find out what has caused the infection and what antibiotic might be used to treat it.

Some infectious agents 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 PCR testing, may be requested in addition to or instead of a routine culture.

How is the sample collected for testing?

Sputum samples may be ‘expectorated’ or induced. Expectorated samples are coughed up and put into a cup provided by the laboratory. The person's mouth should be rinsed with water or a salt solution before 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 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 checked by the laboratory before testing. Successful sputum culture needs good sample collection. A sample that is not "adequate" must be rejected and a further sample collected.

Is any test preparation needed to ensure the quality of the sample?

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