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.
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
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-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.
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
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.