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
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
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
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.
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.
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