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A blood culture is a test to find an
infection in the blood. The blood does not normally have any bacteria or
fungi in it. A blood culture can show what bacteria or
fungi are in the blood.
A 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,
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
chemotherapy) that change how well your body can fight
To test for an infection in the blood, a
sample of blood is collected and placed in a cup with special substances that
allow the bacteria or fungus to grow. The type of bacteria or fungus that grows
is checked with chemical tests and by looking at the culture under a
microscope. Two or three blood samples from different veins are often taken to
make sure a bacteria or fungus is not missed. If no bacteria or fungus grows,
the blood culture is called negative. A blood culture is often done when a
person has a fever because this is the time when the bacteria or fungus is most
likely to have spread to the blood.
blood culture is done to:
You do not need to do anything before
having this test. Tell your health professional if you have recently taken
The health professional drawing blood
Blood 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.
The 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
There is very little chance of a problem from
having a blood sample taken from a vein.
A blood 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.
Bacteria or fungus grows in
the culture. Abnormal culture results are called positive.
If 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.
Reasons you may not be able to
have the test or why the results may not be helpful include:
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 Elsevier.
August 6, 2012
E. Gregory Thompson, MD - Internal Medicine & Joseph O'Donnell, MD - Hematology, Oncology
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