Blood Culture

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

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.

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.