Test Overview

The 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.

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,
kidneys, bowel,
gallbladder, or heart valves.

A blood
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.

Why It Is Done

A
blood culture is done to:

How To Prepare

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

The 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.
  • Put the needle into the vein. More than one needle stick
    may be needed.
  • Attach 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.

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.

How It Feels

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
pinch.

Risks

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.

Results

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.

Blood culture

Normal:

No bacteria or fungus is
found. Normal culture results are called negative.

Abnormal:

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.

What Affects the Test

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.

What 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.
  • A 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 culture.

References

Other Works Consulted

  • 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.

Credits

ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Specialist Medical Reviewer Martin J. Gabica, MD – Family Medicine

Current as ofOctober 9, 2017