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,
whereas 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. Factors such as the amount of
sample collected, the age of the infection, the type of culture done, and
previous use of antibiotics can prevent the growth of bacteria or fungi in the
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. For more information, see
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.