Sputum Culture, Bacterial
- Also Known As:
- Respiratory Culture
- Bacterial Culture
- Bacterial Sputum Culture
- Formal Name:
Test Quick Guide
Sputum is a thick mucus produced in the lungs. It is also known as phlegm and, because of its thickness, can contain infectious germs. In a sputum culture test, a laboratory uses a sample of this mucus to try to encourage the growth of any bacteria or other germs that may be causing an infection.
A sputum culture test can reveal whether there is an infection in the lungs and, if so, the type of bacteria or fungus involved. It is often used in the diagnosis and follow-up care of people with respiratory diseases like pneumonia and tuberculosis.
About the Test
Purpose of the test
The purpose of a sputum culture test is to help determine whether there is an infection in the lungs, and if so, what type of germ causing it. The test is used in the diagnosis, treatment planning, and monitoring of several kinds of lung conditions.
Diagnosis is the process of finding the cause of a patient’s symptoms. A sputum culture test is often involved in diagnosing infections in the respiratory tract. These common infections can be caused by various types of germs, and a sputum culture test can determine if an infection is bacterial or fungal. Sputum cultures are frequently used in the diagnosis of several conditions, including:
- Pneumonia: A sputum culture test can often determine the underlying cause of different types of pneumonia, a kind of lung infection that is a leading cause of death worldwide.
- Tuberculosis: Sputum cultures can detect tuberculosis, which is an infection with a specific bacteria in the lungs that can spread to other parts of the body if left untreated.
- Bronchiectasis: A sputum culture test can also help identify the cause of bronchiectasis, which is a weakening of the airways that reduces the ability of the lungs to function.
Treatment planning for patients with lung conditions may involve the use of a sputum culture test. Knowing the cause of an infection allows doctors to select the most appropriate treatment for a patient. In some cases, a sputum culture may be used for susceptibility testing, which involves testing to see how germs in the sample react when they are exposed to different drugs.
Monitoring is how the health care team tracks a patient’s condition over time. A sputum culture test can help determine if an infection has improved or worsened, which can provide key information about whether a prescribed treatment has been effective.
What does the test measure?
A sputum culture test measures whether bacteria or other germs grow after a sputum sample is placed in a special dish in a laboratory. The dish contains material, known as a medium, that enables any bacteria or fungi present in the sample to grow. The dish is checked after a day or more to evaluate the amount and type of bacteria present.
Most sputum culture tests also involve another test known as a sputum Gram stain. This test applies a dye to the sample, which is then closely analyzed with a microscope. How the dyed sample looks helps to categorize the type of bacteria that are present.
When should I get a sputum culture test?
Sputum culture tests may be prescribed when a respiratory infection is suspected. However, respiratory infections are common and often do not require a sputum culture test. In general, you are more likely to be prescribed a sputum culture test when the following conditions are met:
- You have symptoms of a respiratory infection: While symptoms can vary based on the specific condition, common symptoms of a respiratory infection include a mucus-producing cough, fever, shortness of breath, fatigue, and chest pain when breathing or coughing.
- Imaging tests show a lung infection: In most cases, doctors use a chest x-ray or computed tomography (CT) scan to confirm the presence of a lung infection before recommending a sputum culture test.
- You require hospitalization or are at higher risk of severe disease: If you have severe symptoms that require you to stay in the hospital or if you have other health problems that increase your risk of developing complications from a lung infection, then you are more likely to have a sputum culture test. In contrast, many respiratory infections are mild and sputum culture tests are rarely used for mild cases.
- Your doctor suspects you may have a bacterial or fungal infection: Sputum culture tests are primarily used to identify bacteria or fungi causing an infection. For that reason, these tests are not normally used with respiratory infections like acute bronchitis that are usually caused by viruses.
There are many factors that are taken into consideration when deciding if a sputum culture test is appropriate. For that reason, you should talk with your doctor about your situation and whether a sputum culture test is recommended.
Finding a Sputum Culture Test
How to get tested
A sputum culture test is conducted after it has been prescribed by a health care provider. Sometimes the test is ordered by a doctor who specializes in lung conditions or in infectious diseases.
After being ordered, the test is performed in a medical setting like a doctor’s office, hospital, or medical laboratory.
Can I take the test at home?
There are limited or no options for at-home sputum culture testing. At-home test kits are not currently available.
How much does the test cost?
There is no fixed price for a sputum culture test. Instead, the cost depends on multiple factors including:
- The method used to obtain the sample
- Whether any additional testing was performed on the sample
- Where the test was conducted
- Whether multiple samples were required
Charges for testing can include fees for services provided by the doctor during consultation, the health care provider who collects the sputum sample, and the laboratory that performs the test.
If you have health insurance, these costs may be at least partially paid for by your health insurance plan. To find out more about your costs, including for copays or deductibles, contact your health insurance provider directly.
If you do not have health insurance, talk to your doctor or a hospital administrator about out-of-pocket charges and any programs available to reduce costs for uninsured patients.
Taking a Sputum Culture Test
A sputum culture test requires collecting a sufficient quantity of sputum for laboratory testing. The are two main ways that sputum samples are collected:
- Coughing and spitting into a cup: The most common method of obtaining a sputum sample is by coughing deeply and then spitting the phlegm that comes up into a sample cup. This is generally the least invasive method of sputum culture testing.
- Bronchoscopy: A procedure called bronchoscopy may be used in some situations, such as when coughing doesn’t produce enough sputum for the test or when initial testing is inconclusive. Bronchoscopy involves looking at the inside of the lungs with a camera attached to a flexible tube that is inserted through the nose or mouth. The device is often equipped with a brush or other tool to remove a sputum sample.
Before the test
The preparations for a sputum culture test can vary based on the kind of sample that is being collected.
For a sample obtained by coughing and spitting into a cup, you will often be scheduled for a test in the morning when sputum is more concentrated. The doctor may advise you to drink more water than usual the night before the test to enable you to cough up more phlegm. You may also be told not to eat for an hour or two before the test.
If you are having a bronchoscopy, you will usually need to fast for up to 12 hours before the procedure, which means not eating or drinking anything other than water. Your doctor may adjust your medications prior to testing, especially if you take any blood-thinning drugs. Before the test, it can also be helpful to arrange for help with transportation as well as with everyday tasks for a day or two after the procedure.
Prior to testing, talk with your doctor about the type of sputum culture test that you are having and the preparation that is required. Make sure to carefully follow any pretest instructions provided by your doctor.
During the test
The most common way of collecting a sputum sample is by coughing up phlegm and spitting it into a cup. Before coughing, you are usually instructed to rinse your mouth with water for around 15 seconds. You then breathe deeply a few times and then cough forcefully every couple of minutes until you can produce phlegm that can be spit into a sterile cup.
This process may need to be repeated several times until enough sputum has been collected. To help in this process, the doctor or nurse may:
- Tap lightly on your chest during the test
- Have you inhale a steam or mist that makes it easier to cough up phlegm
Overall, this type of sample collection has few side effects, but it may produce some discomfort from having to cough repeatedly.
A bronchoscopy is a more invasive procedure that is normally done with the aid of local or general anesthesia. During the test, a scope is inserted through your mouth or nose, which may cause you to cough until the anesthesia takes full effect.
Once the scope is inserted, the doctor can see visual images of the inside of your lungs and airway. They can also use the device to collect a sample of sputum or to wash sputum out of the lungs with a saline solution.
During a bronchoscopy, you may experience some pressure from the scope or feel some difficulty breathing. However, the procedure is rarely painful because of the anesthesia that is used.
After the test
There are not usually any lasting effects after providing a sputum sample by coughing up phlegm and spitting it into a cup. If you continue coughing after the test, you may experience some discomfort.
A bronchoscopy can be an outpatient procedure but sometimes requires a brief stay in the hospital. After the test, it may take a couple of hours to regain your normal ability to cough. During that time, you cannot eat or drink anything.
You may have some discomfort that continues after a bronchoscopy. For example, it is normal for your throat to feel irritated or scratchy for a few days. If this problem persists or if you notice any other effects of the procedure, you should contact your doctor.
Sputum Culture Test Results
Receiving test results
After arriving at the laboratory, the sputum sample is applied to a dish containing a medium that allows bacteria in the sample to grow. To give bacteria time to grow, the culture is not normally analyzed until at least 24 hours have passed. As a result, it can take a few days for a sputum culture test result to be available.
The results of a sputum culture will normally be provided by a member of your health care team. Usually these results will be discussed with you while you are at the doctor’s office or hospital, but results may be given over the phone. A laboratory test report may also be sent by mail or accessed through an online health portal.
Interpreting test results
The results of a sputum culture test are primarily reported as normal (negative) or abnormal (positive):
- Normal or negative means that the test did not detect any harmful germs in your sputum.
- Abnormal or positive indicates that potentially harmful bacteria or fungi were detected. The test report names the germ or germs that were found and the approximate amount of germs present. This is shown either numerically from +1 to +4 or with terms like rare, few, moderate, or abundant.
Abnormal findings on a sputum culture may support a diagnosis of a lung condition like pneumonia, tuberculosis, or bronchiectasis that is tied to an underlying infection. However, a normal or abnormal result is not a definitive diagnosis and must be interpreted carefully by your doctor.
A sputum culture test result is not evaluated in isolation. It is always reviewed in the context of several other factors, such as:
- Your symptoms and health condition
- The type and quantity of bacteria present
- The results of other tests, such as a sputum Gram stain
- The quality of the sample you were able to provide
- Whether or not you are hospitalized
Although a positive result shows that certain bacteria or fungi are present, this does not necessarily prove that those germs are the cause of respiratory symptoms. In addition, some respiratory germs that can cause lung conditions may fail to grow in the laboratory culture test.
As a result, your doctor will consider whether the results of the sputum culture test fit with the symptoms you are experiencing and with the findings of other tests. If there is a discrepancy or unexpected result, it may warrant additional testing, including the possibility of a repeat sputum culture test using a sample obtained through a bronchoscopy instead of coughing up phlegm.
In light of the numerous factors that must be considered when interpreting a sputum culture test, it is essential to talk to your doctor about your test result in order to understand its significance in your specific situation.
Are test results accurate?
Accredited laboratories follow careful procedures to help make sputum culture tests accurate and generally dependable for doctors. But no test is perfect, and there are issues that can affect the accuracy of a sputum culture test:
- Insufficient sputum sample: Sputum is distinct from saliva or mucus produced in the upper part of your airway, but samples from coughing may contain more of these other fluids than actual sputum. A lack of sputum in the sample can reduce the accuracy of testing.
- Sample contamination from the mouth: In a sample produced by coughing and spitting, it is possible for bacteria in the mouth to contaminate the sample, which is why you usually rinse your mouth before the test.
- Use of antibiotics: Antibiotics can directly affect bacterial growth, so sputum culture test results may be less accurate in people who are taking antibiotics.
If you have questions about the accuracy of your results, talk to the doctor who prescribed your sputum culture test for more information.
Do I need follow-up tests?
The need for follow-up tests depends on the kind of respiratory disease you may have, your symptoms, the type of sputum culture sample you provided, whether any bacteria or fungi were found, and the results of other tests.
If the laboratory determined that your sample was unsuitable or insufficient for testing, you may need to provide another sputum sample. In some cases, a bronchoscopy may be recommended to look at your lungs and directly obtain sputum for testing.
Other types of laboratory tests may be used to support or confirm the findings of a sputum culture test. In addition, an inconclusive or unexpected result may warrant additional tests to better understand your situation. For example, for tuberculosis culture testing that is inconclusive, a biopsy to remove a sample of lung tissue may be needed to confirm a diagnosis.
Questions for your doctor about test results
To better understand the significance of your sputum culture test, you can bring up one or more of the following questions with your doctor:
- Were any abnormal germs found on my test?
- Do the findings of this test affect my diagnosis or treatment plan?
- Do you recommend any follow-up tests based on my sputum culture test result?
A sputum culture test is not the only method of finding germs in the lungs. The following sections address how the sputum culture test compares to several related tests.
A sputum culture test and a sputum Gram stain
A Gram stain is a way of detecting bacteria and is often performed on the same sample as a sputum culture test.
In a Gram stain, different colored materials are applied to the sample, which is then analyzed under a microscope. Based on the change in color and the shape and size of cells, the test can detect certain categories of bacteria. A Gram stain can also be used to help determine whether a sputum sample is suitable for culture.
A sputum Gram stain can be completed more quickly than a culture test, but it generally does not allow for the identification of the specific type of bacteria causing the infection. For this reason, a sputum culture test is typically conducted to confirm and build upon the findings of a sputum Gram stain.
Comparing a sputum culture test and a sputum fungal smear
A sputum fungal smear is different from a sputum culture test because it looks for fungi under a microscope instead of trying to grow fungi in a dish.
Comparing a sputum culture test and a sputum PCR
Polymerase chain reaction (PCR) is a testing technique that looks for traces of genetic material in a test sample. A sputum PCR is most frequently used to test for viral infections such as influenza, COVID-19, or other respiratory viruses.
Sputum culture tests take longer than PCR tests but are more frequently used for finding bacterial and fungal infections. For some conditions, like tuberculosis, a sputum culture test is considered to be more useful when planning treatment.
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