Sputum Culture

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Also known as: Respiratory Culture
Formal name: Culture, sputum

The Test Sample

What is being tested?

Sputum cultures detect the presence of disease-causing bacteria in those who have bacterial pneumonia and other lower respiratory tract infections. 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 (bronchi and lungs); 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 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.

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 check that the sample adequate. If a sample contains a lot of normal cells that line the mouth (squamous epithelial cells), then the sample is not adequate for culture and a re-collection of the sample may be required. If the sample contains mainly white blood cells from the body’s response to an infection, then the sample is adequate for culturing.

Once a sputum sample has been accepted, it is placed on or in appropriate nutrient media and incubated under special conditions. 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 disease-causing bacteria present.

The next step is to identify the different types of bacteria present and categorise them as normal or potential disease-causing bacteria. Identification is a step-by-step process that may involve several biochemical tests and observations of how the bacteria grow.

'Antimicrobial susceptibility testing’ is frequently used to identified disease causing-bacteria and find out whether they are likely to respond to particular antibiotics.

The sputum culture, Gram stain(s), and susceptibility testing all contribute to helping the doctor find out what has caused the infection and what antibiotic might be used to teat 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 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 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.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

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.