Blood Culture

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 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 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.

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 an illustration of endocarditis.
• Find the best antibiotics to kill the bacteria. This is called sensitivity testing.
• Find the cause of an unexplained fever or shock or a person becoming extremely ill.

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

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.

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.

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.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken. Firm pressure may be needed on the puncture site.

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.

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:

• 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 are spilling into 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.

• 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). Therefore, 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.

Other Works Consulted

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.