A sputum culture is
a test to detect and identify bacteria or
fungi (plural of fungus) that are infecting the lungs
or breathing passages. Sputum is a thick fluid produced in the lungs and in the
airways leading to the lungs. A sample of sputum is placed in a container with
substances that promote the growth of bacteria or fungi. If no bacteria or
fungi grow, the culture is negative. If organisms that can cause infection
grow, the culture is positive. The type of bacterium or
fungus will be identified with a microscope or by chemical tests.
If bacteria or fungi that can cause infection grow in the culture, other
tests may be done to determine which antibiotic will be most effective in
treating the infection. This is called susceptibility or
This test is done on
a sample of sputum that is usually collected by coughing. For people who can't
cough deeply enough to produce a sample, they can breathe in a mist solution to
help them cough.
Why It Is Done
A sputum culture is done to:
Find and identify bacteria or fungi that are
causing an infection (such as
tuberculosis) of the lungs or the airways leading to
the lungs. Symptoms of a lung infection may include difficulty breathing, pain
when breathing, or a cough that produces bloody or greenish brown
Identify the best
antibiotic to treat the infection (sensitivity
Monitor treatment of an infection.
How To Prepare
Do not use mouthwash before collecting a
sputum sample because it may contain antibacterial substances that could affect
bronchoscopy will be used to collect your sputum
sample, your doctor will tell you how soon before the test to stop eating and
drinking. Follow the instructions exactly about when to stop eating and drinking, or your surgery may be canceled. If your doctor has instructed you to take your medicines on the day of surgery, please do so using only a sip of water.
Tell your doctor
if you have recently taken any antibiotics.
How It Is Done
Usually, the sputum sample is collected
early in the morning before you eat or drink anything. In some cases
(especially if you may have tuberculosis), three or more morning samples may be
If you wear dentures, you will need to remove them before
collecting a sputum sample. Then rinse your mouth with water, take a deep
breath, and then cough deeply to produce a sample of sputum. The health
professional collecting the sample may tap on your chest to help loosen the
sputum in your lungs before you cough. If you still have trouble coughing up a
sample, you may be asked to inhale an aerosol mist to help you cough.
Some people may need bronchoscopy to collect a sputum sample. During
bronchoscopy, a thin, lighted tube (bronchoscope) is inserted through your
mouth or nose into the airways leading to your lungs. You will be given
medicine that numbs your throat and nose so you do not feel discomfort from
the bronchoscope. You may also be given a sedative to make you sleepy during
the procedure. To collect the sputum sample, a salt solution may be washed into
the airway and then suctioned into a container. A small, thin brush may be used
to collect a sample.
A sputum sample can also be collected using
suction. During this procedure, a soft, flexible tube (called a nasotracheal
catheter) is inserted through the nose and down the throat. Suction is applied
for up to 15 seconds to collect the sputum sample. This method of collecting a
sputum sample is often used for people who are very sick or unconscious.
After a sample is collected
sputum sample is collected, it will be placed in a container with substances
(growth medium or culture medium) that promote the growth of infecting
organisms (bacteria or fungi). Bacteria usually need 2 to 3 days to grow. Fungus often takes a week or longer to grow. The organism that causes
tuberculosis may take 6 weeks to grow. Any bacteria or fungi that grow will be
identified under a microscope or by chemical tests. Sensitivity testing, to
determine the best antibiotic to use against the organism that grows, often
takes 1 to 2 more days.
How It Feels
If you have discomfort when taking a deep
breath or when coughing, obtaining a sputum sample may be uncomfortable. If you
need to inhale the aerosol mist to produce a sputum sample, you will often feel
a deep, uncontrollable urge to cough.
During bronchoscopy or
collection of a sputum sample using a catheter, you may feel a strong urge to
cough as the bronchoscope or catheter passes into the back of your throat. You
may also feel as if you cannot breathe. Try to relax and breathe slowly while
the bronchoscope or catheter is in place. If you are given medicine to numb
your throat and nose, you may feel as if your tongue and throat are swollen and
that you cannot swallow.
Your throat may feel sore following bronchoscopy
or collection of a sputum sample using a nasotracheal catheter.
you have severe
bronchitis, you may find it hard to breathe
during collection of a sputum sample using a nasotracheal catheter.
A sputum culture is a test to detect and
identify bacteria or
fungi that are infecting the lungs or breathing
passages. Some types of bacteria or fungi grow quickly in a culture and some
grow slowly. Test results may take from one day to several weeks, depending on
the type of infection suspected. Some organisms (such as Chlamydophila pneumoniae and mycoplasma) do not grow in a
standard culture and need a special growth medium to be detected in a sputum
Sputum that has passed through
the mouth normally contains several types of harmless bacteria, including some
types of strep (Streptococcus) and staph (Staphylococcus). The culture should
not show any harmful bacteria or fungi. Normal culture results are
Harmful bacteria or fungi are
present. The most common harmful bacteria in a sputum culture are those that
tuberculosis. If harmful bacteria or fungi grow, the culture
If test results point to an infection, sensitivity testing
may be done to determine the best antibiotic to kill the bacteria or fungus.
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
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
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.
Adam Husney, MD - Family Medicine
Robert L. Cowie, MB, FCP(SA), MD, MSc, MFOM - Pulmonology
How this information was developed to help you make better health decisions.