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Sputum Culture

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Recent use of antibiotics, which may prevent the growth of bacteria or fungi in the culture.
  • Contamination of the sputum sample.
  • An inadequate sputum sample.
  • Waiting too long to deliver the sample to the laboratory.
  • Use of mouthwash before collecting a sputum sample.

What To Think About

  • Organisms (such as bacteria or a fungus) may be identified by using special dyes on the sputum sample. This is called a Gram stain. A Gram stain can help a doctor:
    • Determine whether the sputum sample is adequate for culture. For example, a sputum sample that is not collected properly may not contain enough bacteria to warrant a culture, or the sample may be contaminated by other bacteria that are not infecting the lungs. If the sample is not adequate for culture, another sample can be obtained.
    • Make an initial diagnosis before the culture results are received. In some cases, Gram stain results can be available within 30 minutes, but culture results may not be available for one to several days. Information received from a Gram stain can help your doctor treat the infection earlier, rather than waiting for the culture results.
  • Because sputum collected by coughing usually contains bacteria from your mouth, these culture results must be considered along with your symptoms, condition, and other test results, such as a chest X-ray.
  • A culture that does not grow any bacteria or fungi may not rule out an infection.
  • A special growth medium is needed for some cultures, such as for those that can find infection by tuberculosis (TB) bacteria, mycoplasma, Chlamydophila pneumoniae, or fungus. TB bacteria and most types of fungi grow very slowly and may not show up in a culture for several weeks. So treatment for a suspected fungus or TB infection may begin before culture results are known.
  • Sensitivity testing helps a doctor choose the best medicine to kill specific types of bacteria or fungi infecting a person. Differences in the genetic material (DNA) of bacteria or fungi may make them resistant to certain antibiotics. In such cases, those antibiotics can't kill all of the bacteria. When an effective antibiotic is chosen, you must complete the entire course to lower the chance that the bacteria will become resistant to the antibiotic. Stopping antibiotic treatment early kills only the most sensitive bacteria, while those that are more resistant can multiply and prolong the infection. Subsequent infections may also be harder to treat if caused by resistant bacteria.
  • Bronchoscopy is often done if a serious or recurring lung infection is present, especially when other tests have not definitely determined the cause. To learn more, see the topic Bronchoscopy.

WebMD Medical Reference from Healthwise

Last Updated: November 01, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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