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Bacterial Sputum Culture

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Also known as: Respiratory Culture; Bacterial Culture, respiratory; Sputum Culture, routine bacterial
Formal name: Culture, sputum

Were you looking instead for tests performed on sputum to detect fungi [see Fungal Tests] or mycobacteria (tuberculosis) [see AFB Smear and Culture]?

The Test Sample

What is being tested?

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 from the upper respiratory tract. Sputum samples may be expectorated or induced (See the section below on sample collection.)

Bacterial sputum cultures detect the presence of disease-causing bacteria (pathogens) in people who are suspected of having bacterial pneumonia or other lower respiratory tract infections. Bacteria in the sample are identified and susceptibility testing is performed to guide antibiotic treatment.

Sometimes a respiratory infection is caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture. 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.

Typically, the first step in the routine analysis of a sputum sample is a Gram stain to identify the general type of bacteria that may be present. The sample is then 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. That means that when a person has a bacterial respiratory infection, there will typically be harmless bacteria that are normally present in the mouth, throat, etc. (normal flora) as well as disease-causing (pathogenic) bacteria present.

A trained laboratorian differentiates normal flora from pathogenic bacteria and identifies the various types of bacteria present in the culture. Identification is a step-by-step process that may involve several biochemical, immunological, and/or molecular tests tests and observations of the organism's growth characteristics.

Antimicrobial susceptibility testing is frequently required to guide the treatment 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 health practitioner which pathogen(s) are present and which antibiotic therapies are likely to inhibit their growth. 

How is the sample collected for testing?

Sputum samples may be coughed up or induced. Samples that are coughed up are expelled into a sterile cup provided by the laboratory. 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 following instructions provided and inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs. Steam inhalation or a hot shower can also be useful in loosening the phlegm. Sometimes, induction of sputum might be assisted by a respiratory therapist technician.

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. If examination of a Gram stain of the sample reveals that it 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 body's response to an infection, then it is considered to be an adequate sample for culturing.

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?

You may be instructed to brush your teeth and rinse your mouth with water prior to sample collection. You may also be instructed to avoid food for at least 1-2 hours before the sample is collected, which is usually first thing in the morning.