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Blood Culture

Definition

The purpose of a blood culture is to see if infectious organisms are living in a person's blood.

Who is a candidate for the test?

Blood cultures are performed whenever a healthcare provider suspects that a person's symptoms are due to an infection in the blood. Normally, blood should not contain organisms.

How is the test performed?

In order to do a blood culture, a sample of blood is required. This sample of blood is usually obtained through a vein in the hand or arm. A thin piece of rubber is wrapped tightly around the arm, causing the vein to swell with more blood. The skin over the vein is carefully cleaned to remove the bacteria that are normally present on the skin.
A small needle is then inserted through the skin and into the vein. Once the needle is in the vein, blood is allowed to flow through the needle and into a test tube. The blood sample is then sent to the lab so cultures can be done.
A culture is an attempt to get organisms that may be present in the blood to grow and multiply. To do this, the blood sample is placed in specially prepared containers and warmed at different temperatures. When enough organisms grow and multiply, they can be identified. This process usually takes 1 to 3 days. In some cases, a culture may take weeks to develop. The organism identified may be a virus, bacteria, fungus, or another type of organism.
The healthcare provider will use blood culture results to select the appropriate medicine. Often, treatment is started before the culture results are known. The provider may need to switch medicines once the culture results come back.

What is involved in preparation for the test?

No preparation is needed for a blood culture.

What do the test results mean?

Normally, blood does not contain organisms. If the blood culture is "negative," no organisms have grown in the specially prepared containers.
In some cases, a person may have a blood infection, but the test is still negative. This may occur because the bloodstream is being intermittently "showered" with bacteria from another infection source in the body. Repeated blood culture tests may succeed in "catching" the bacteria. Or, the organism may never be identified because it failed to survive or grow in the specially prepared containers.
In some cases, the organism that grows is called a "contaminant." This means the blood sample contains organisms that did not originate from the tested person's blood. For example, bacteria normally present on the skin could have entered through the needle when the sample of blood was taken. In this case, an infection is not actually present in the person's blood.
If an infection of the blood is truly present and shows up in the culture, valuable information is gained. Every organism is different and responds to different treatments. Each positive blood culture is tested to see which medicines will be effective to kill the organism. One organism may respond to a certain antibiotic while another organism may not.

Sources

Harrison's Principles of Internal Medicine, 1998, Fauci et al.

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