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What is a Blood Culture?

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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 16 September 2016
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A blood culture is part of blood test that can help determine whether a person has bacteria or other infectious agents in his or her bloodstream. Blood cultures may be required when blood infection like septicemia is suspected, or when it’s believed that infection exists in hard to scan areas. For instance if people have bacterial endocarditis, and bacterial matter is not showing up on x-rays or on echocardiograms, doctors might use a blood culture to determine its presence, since some bacteria will spread into the blood stream from the heart.

For people having a blood culture, the procedure is fairly simple, but it does involve at least two blood draws, that are usually drawn from different areas of the body, for example opposite arms. Each arm is swabbed with alcohol to prevent the skin from contaminating the blood sample with normal skin bacteria. The samples taken are then treated specially to see if they will grow bacteria.

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This usually means placing the samples in a machine that keeps them at body temperature, so if bacteria is present, it will continue to grow. Samples are then monitored for up to five days to determine if bacteria exist. At the same time, or after a positive blood culture, meaning presence of bacteria, doctors must figure out what type of bacteria is present to determine how best to treat it. They may use a process called subculturing, where they grow bacteria on special plates with agar to determine its type.

Time it takes to determine if a blood culture is positive can vary. Samples may be kept for up to five days to make sure that nothing appears to “grow” in the blood taken. Usually, if bacteria is present in the body, and has been caught in the sample, which isn’t always the case, it takes a couple of days before the blood culture can be determined positive. Sometimes negative blood cultures aren’t accurate, and people may need to have several tests over several days if doctors reasonably suspect bacterial or fungal infection in the blood stream.

There are people who are more likely to have a blood culture performed. Infants and the elderly are more susceptible to blood infections because they have weaker immune systems. Also, those with immunosuppressive diseases like lupus or AIDs might be automatically more suspect of having blood infections if they present with symptoms like high fever, chills, and body aches. However, these symptoms aren’t always indication of blood infection and might instead signal that someone has a nasty virus like the influenza virus. Moreover anyone at any age can develop blood infection.

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