When you have symptoms associated with a lower respiratory tract infection; when you have been treated for bacterial pneumonia
Rinse mouth out with water prior to collection
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.
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.
How is it used?
A sputum culture is requested to detect and diagnose bacterial infections such as bacterial pneumonia. A bacterial infection can reach the lungs in several ways. Bacteria may spread from the mouth and throat to upper respiratory tract, bacteria in oral or gastric secretions may be breathed into the lungs as droplets in the air. These droplets are produced when a person sneezes or coughs and can pass into the lungs. Bacteria can also spread to the blood (septicaemia) from a local infection and then be carried to the lungs. Bacterial pneumonia may be a person's main infection, or it may develop after a viral infection such as influenza, a cold or viral pneumonia.
Anyone can get a bacterial respiratory infection, but the elderly, those with suppressed immune systems, those with damaged lung tissue, those who are exposed to lung irritants, such as through smoking, and those with diseases that affect the lungs, such as cystic fibrosis, are at increased risk.
A sputum culture may be used by itself, with a FBC (Full Blood Count) to evaluate the type and number of white blood cells as an indication of infection, and/or with a blood culture to test for septicaemia.
If disease-casuing bacteria are found during a sputum culture, then antimicrobial susceptibility testing is usually performed so that the appropriate antibiotics can be prescribed.
When is it requested?
A sputum culture is requested when a doctor suspects that a person has a bacterial infection of the lungs or airways, such as bacterial pneumonia, which may show as changes in the lungs as seen on a chest x-ray. Symptoms may include:
- Fever, chills
- Muscle aches
- Trouble breathing
- Chest pain
Sometimes a sputum culture may be requested after treatment of an infection, to check its effectiveness.
What does the test result mean?
If disease-causing bacteria are detected in a person with signs and symptoms of a lower respiratory tract infection, then it is likely that the person's symptoms are due to a bacterial infection. The most common cause of bacterial pneumonia in adults in the UK is Streptococcus pneumoniae (pneumococcus). Other common bacteria include:
- Staphylococcus aureus (staph)
- Haemophilus influenzae
- Moraxella catarrhalis
- Klebsiella pneumoniae
- Streptococcus pyogenes
If disease-causing bacteria are not detected with a culture, then it may be that the person's symptoms are due to a viral infection, or that the pathogen was not present in sufficient quantity in the sample collected for it to be detected. It may also be due to the fact that the microorganism responsible is not detectable with a routine bacterial culture. Examples of organisms NOT detected with a routine bacterial culture of the sputum include:
- Mycoplasma pneumoniae and Chlamydia pneumonia - common in young adults
- Legionella species
- Mycobacterium tuberculosis (tuberculosis)
- Pneumocystis jiroveci - a fungus that can be seen in those who have had organ transplants or who have HIV/AIDS
- Other fungi
- Rarely, a parasitic infection may also cause symptoms of a lower respiratory infection.
Is there anything else I should know?
Those people whose lungs have become damaged, through disease, exposure to toxins or chronic exposure to irritants, or from previous infections, have an increased risk of recurrent infections.
With bacterial respiratory infections, the sputum may have a thick consistency (viscous), appear discoloured – yellowish, greenish, greyish, or rarely rusty or bloody – and may have an unpleasant odour.
Can bacterial pneumonia be prevented?
There is a pneumococcal vaccine that helps protect against invasive Streptococcus pneumoniae (pneumococcus) infections, the most common cause of bacterial pneumonia in adults. There is also a different pneumococcal vaccine for young children that protects them against serious infections with these bacteria, and there is a vaccine for infants to protect against serious infections with Haemophilus influenza type b.
Why would my doctor collect more than one sputum sample?
This may be done if the first sample is not considered adequate by the laboratory or if the doctor wants to request additional tests on sputum samples.
Why would a separate test be requested for fungus or an AFB smear and culture?
I have been sick for more than a week. Why would my doctor request a sputum culture now?
Once I have been treated for a bacterial lower respiratory tract infection, can the infection return?
For most people, once the infection has been successfully treated, it will not return. If the treatment was not successful, then the infection may remain or re-emerge. If someone has a condition that increases their risk of lung infection, then they may have new infections. In some cases, these recurring infections are difficult to treat.
On This Site
Tests: Gram Stain, Susceptibility Testing, AFB Smear and Culture, Blood Culture, Fungal Tests, FBC
Conditions:Pneumonia, Lung Diseases, Tuberculosis, Nontuberculous Mycobacteria, Fungal Infections, Cystic Fibrosis