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

One of several different types of blood culture checks for organisms called aerobic bacteria. Bacteria are tiny organisms that can live in both the human body and our environment. The aerobic type of bacteria can live and grow only where there is oxygen.

Some aerobic bacteria cause illness, while others pose no problems to humans or may be helpful. Even normal bacteria, however, can become harmful if certain conditions allow them to:

• move from the part of the body where they usually live to other parts where they should not be found
• multiply quickly

Who is a candidate for the test?

This test is done most often with very young or old people or with those who have weakened immune systems. However, it may be used any time a person has a serious infection. This is because most severe infections, especially those from the lungs or kidneys, can spread to the blood.

• fast heartbeat, known as tachycardia
• fever with or without chills
• low blood pressure
• nausea and vomiting

Other symptoms often arise from the original site of the infection. For example, a person with a lung infection may have a cough.

How is the test performed?

Blood samples for this test are usually taken from veins in the forearm or the back of the hand. Samples may be taken from two different sites to increase the chance of detecting bacteria in the blood. Testing two sites also helps to rule out contamination of the test by bacteria from the skin or from another source.

Two or more blood samples may be collected from each site so both aerobic and anaerobic bacteria can be detected. Anaerobic bacteria can live and grow without oxygen, and some may even die when exposed to oxygen.

First, a band is tied around the upper arm to slow the circulation. This enlarges the veins below. A puncture site is selected and cleaned. Next, a needle is inserted into a vein. Blood is collected and placed into a vial containing special nutrient substances that help the bacteria to grow. The needle is removed from the person's arm, and the vial is sent to the lab. A bandage is put on the puncture site to stop any bleeding.

In the lab, the vial is held at a specific temperature and watched to see if bacteria grow. It takes from 24 to 72 hours or longer for aerobic bacteria to grow. If bacteria grow, the lab can identify them using special tests.

What is involved in preparation for the test?

No preparation is needed for this test.

What do the test results mean?

Blood does not normally contain bacteria. If any bacteria are found with this test, the result is abnormal. This is called a positive test or a positive blood culture. A positive test generally means one of two things:

• The person has bacteria in his or her blood, a condition known as sepsis. Bacteria may also get into the blood through an open wound, through the use of intravenous drugs, or on an artificial device inserted into the bloodstream. In most people, the infection started in another part of the body, such as the lungs or kidneys.
• Rarely, the blood sample was contaminated with bacteria that live on the skin. As a needle is inserted through the skin to collect blood, bacteria from the skin may get on the needle. To avoid this problem, the skin is cleaned before inserting a needle. Bacteria that cause contamination are often different from the bacteria that cause serious infections of the blood.

If a person has a positive test from contamination, no treatment is required. If a person has bacteria in his or her blood, antibiotic treatment is needed. This test allows the bacteria that are causing the infection to be identified and treated. For example, an antibiotic may kill one type of bacteria and be totally ineffective against another type.

Examples of aerobic bacteria that may cause blood infections include:

• Certain Neisseria
• Pseudomonas aeruginosa
• Staphylococcus, known as "staph" for short

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.