blood does not normally have any bacteria or fungi in it. A blood culture is a test of a
blood sample to find germs (such as bacteria or a fungus) that can cause an infection.
bacterial infection in the blood, called bacteremia, can be serious because the blood
can spread the bacteria to any part of the body. A blood infection most often occurs
with other serious infections, such as those affecting the lungs, kidneys, bowel, gallbladder, or heart valves.
infection may also develop when the immune system is weak. This can occur in infants
and older adults, and from disease (such as cancer or AIDS) or from medicines (such
as corticosteroids or chemotherapy) that change how well your body
can fight infections (immunity).
For a blood culture, a sample of blood is added
to a substance that promotes the growth of germs. The type of germ may be identified
using a microscope or chemical tests. Sometimes other tests are done to find the right
medicine for treating the infection. This is called sensitivity testing. Two or three blood samples from different veins are often
taken to make sure a bacteria or fungus is not missed.
Find the cause of an unexplained fever or shock or a person becoming extremely ill.
You do not need to do anything before having this test. Tell your
health professional if you have recently taken antibiotics.
How It Is Done
health professional drawing blood will:
Wrap an elastic band around your
upper arm to stop the flow of blood. This makes the veins below the band larger so
it is easier to put a needle into the vein.
Clean the needle site carefully
with alcohol or iodine so skin bacteria will not get in the blood sample.
the needle into the vein. More than one needle stick may be needed.
a tube to the needle to fill it with blood.
Remove the band from your arm
when enough blood is collected.
Put a gauze pad or cotton ball over the needle
site as the needle is removed.
Put pressure to the site and then a bandage.
is often collected from two or three different body sites. Or it may be collected
at two different times a few hours apart.
Some people may have long-term catheters
placed in a major vein because they are receiving chemotherapy or nutrition supplements
for weeks or months at a time. For these people, blood for a blood culture will be
collected from their catheters for this test.
How It Feels
blood sample is taken from a vein in your arm. An elastic band is wrapped around your
upper arm. It may feel tight. You may feel nothing at all from the needle, or you
may feel a quick sting or pinch.
There is very little chance
of a problem from having a blood sample taken from a vein.
You may get
a small bruise at the site. You can lower the chance of bruising by keeping pressure
on the site for several minutes.
In rare cases, the vein may become swollen
after the blood sample is taken. This problem is called phlebitis. A warm compress
can be used several times a day to treat this.
culture is a test to find an infection in the blood. Most bacteria can be seen in
the culture in 2 to 3 days, but some types can take 10 days or longer to show up.
Fungus can take up to 30 days to show up in the culture.
No bacteria or fungus is found. Normal
culture results are called negative.
or fungus grows in the culture. Abnormal culture results are called positive.
bacteria are found in the culture, another test is often done to find the best antibiotic that will kill the bacteria. This
is called sensitivity or susceptibility testing. Sensitivity testing is important so the
blood infection is treated correctly. This also helps prevent bacteria from becoming
resistant to antibiotics.
What Affects the
Reasons you may not be able to have the test or why the results may not
be helpful include:
If you have taken antibiotics recently. These medicines
may stop the growth of bacteria in the culture.
If the blood sample is contaminated
by bacteria or fungus on the skin.
If the blood test misses the time when
bacteria actually are in the blood. Blood culture tests are done at several different
times to make sure bacteria are not missed.
If the blood test is not done
correctly or the blood sample is not processed properly. In these cases, a false-positive or false-negative result could occur.
To Think About
Some types of bacteria infect the blood when another infection
of the kidneys, throat, lungs, or another part of the body is present. This may not
mean a serious infection of the blood.
About 5% of blood cultures are contaminated
with normal skin bacteria (a type of staph bacteria). So it is sometimes hard to see
whether the bacteria that grow in the culture are the cause of the blood infection
or not. This is why more than one blood sample is taken. When the same bacteria grow
in several blood cultures, it is likely that those bacteria are in the blood and are
causing the infection. When staph bacteria grow in the culture in less than 48 hours,
it is likely that the staph bacteria are in the blood and are causing the infection.
culture that does not grow any bacteria does not always mean a blood infection is
not present. The amount of blood taken, the timing of the blood sample, the type of
culture done, and recent use of antibiotics can affect the growth of bacteria in the
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic
Procedures, 5th ed. St. Louis: Saunders.
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 Elsevier.
ByHealthwise Staff Primary
Medical ReviewerE. Gregory Thompson, MD - Internal
Medicine Adam Husney, MD - Family Medicine Martin J. Gabica, MD - Family Medicine
This information does not replace the advice of a doctor.
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