Sputum Culture, Bacterial

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.

Sputum culture test vs. 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.

Sputum culture test vs. 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.

Sputum culture test vs. 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.