Blood normally doesn't 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. That's 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. It can happen because of a 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 found 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 that a bacteria or fungus is not missed.
Why It Is Done
A blood culture is done to:
- Find a bacterial infection that has spread into the blood, such as meningitis, osteomyelitis, pneumonia, a kidney infection, or sepsis. A culture can also show what type of bacteria is causing the infection.
- Find a fungal infection, such as yeast, in the blood.
- Check for endocarditis, which is an infection of the valves of the heart.
- Find the best antibiotics to kill the bacteria or fungi. This is called sensitivity testing.
- Find the cause of an unexplained fever or shock or a person becoming extremely ill.
How To Prepare
- In general, there's nothing you have to do before this test, unless your doctor tells you to.
- Tell your health professional if you took antibiotics recently.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
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
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
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.
Current as of: June 17, 2021
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine