At a glance
Also known as
Respiratory Culture; Sputum MC&S
Why get tested?
To detect and identify the cause of lower bacterial respiratory tract infections and pneumonia or to monitor the effectiveness of antibiotic treatment
When to get tested?
When you have symptoms associated with a lower respiratory tract infection; when you have been treated for bacterial pneumonia
A fresh sample (deep respiratory secretions, not saliva from the mouth), usually collected first thing in the morning into a sterile container.
Test preparation needed?
Rinse mouth out with water prior to collection
What is being tested?
A sputum culture detects the presence of in those who have bacterial pneumonia or a lower respiratory tract infections. Pathogenetic bacteria in the sputum sample are identified and susceptibility testing is performed to help guide apprpriate 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 from the mouth. Care must be taken in the sample collection process to ensure it is from the lower airways and not the mouth. If a sample is mostly saliva, the grown in 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 potential pathogenic bacteria infecting the lungs.
The first step in the analysis of a fresh sputum sample is a Gram stain to identify the type of bacteria present and to determine sample adequacy. If a sample contains a significant number of epithelian (skin) cells that line the mouth, lots of normal oral bacteria and no white blood cells, then the sample is not generally considered adequate for culture and a re-collection of the sample may be required if clinically relevant. If the sample contains a high amount of white blood cells that indicate an infection, then it is considered to be an adequate sample for culturing. Sputum Gram stain can also help identify aspiration pneumonia along with clinical findings.
Once a sputum sample has been accepted, it is placed on or in appropriate and incubated. The media and environment conditions encourage bacteria growth, this then allows for further testing and indention. Because sputum is not sterile, cultures will grow both as well as and any bacterial pathogens that are present.
Identification is a step-by-step process that may involve several biochemical tests and observations of the organism's growth characteristics. Culture results are reported in correlation to the Gram stain, aiding the laboratory to identify whether there is an infection present.
Antimicrobial susceptibility testing is frequently performed to guide the treatment of any identified pathogens and to test whether the pathogen will respond to specific antibiotics. The sputum culture, Gram stain(s), and susceptibility testing all contribute to a report which informs the doctor which pathogen(s) are present and what antibiotic therapy is likely to eradicate infection.
The infection may be caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture, so other tests, such as an Acid fast bacilli (AFB) culture, legionella culture, fungal culture, or PCR, may be ordered in addition to or instead of a routine culture. If an unusual pathogen is suspected the laboratory should be informed to ensure the correct cultures and/or extra tests are completed.
How is the sample collected for testing?
Sputum samples may be expectorated or induced. Expectorated samples are coughed up and expelled into a sterile container. The person's mouth should be rinsed with water or saline prior to sample collection. Deep coughing is generally required, and the patient 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. Depending on the laboratory, sputum samples may be evaluated and accepted by the laboratory before they are processed fro culture. Useful sputum culture results rely heavily on good sample collection. A sample that is not considered "adequate" may be rejected and a recollection requested. an adequate sputum sample means that it is considered representative of conditions in the lower respiratory tract i.e. not heavily contaminated with saliva.
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 mouth.