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, 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).

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.

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 bacteria living on the valves of the heart. See a picture of endocarditis.
• 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

You do not need to do anything before having this test. Tell your health professional if you have recently taken antibiotics.