Alternative Names blood culture for aerobic bacteria
Definition A test called a blood culture is done to see if there is an infection of the blood. There are different types of blood culture tests. One of these tests checks for a type of organism called aerobic bacteria.
Bacteria are tiny organisms that live in the body and the environment. Aerobic bacteria are bacteria that can live and grow in the presence of oxygen.
Some bacteria cause illness. Others pose no problems to humans or may be helpful. But even harmless bacteria can become harmful if certain conditions allow them to:
multiply quickly
move from the part of the body where they usually live
Who is a candidate for the test? This test is done most often on very young or old people and people with weakened immune systems. However, this test may be done anytime a person has a serious infection. This is because most severe infections can spread to the blood. For example, kidney and lung infections are two of the most common causes of blood infections.
Other symptoms are often due to the underlying infection that has spread to the blood. 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 back of the hand. The samples may be taken from two different sites. This increases the likelihood of detecting bacteria in the blood. This can also help 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 aerobic and anaerobic bacteria can both 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. This enlarges the veins below. The puncture site is cleaned. A needle is then inserted into a vein. Blood is collected and placed into a vial. The vial contains special food to allow the bacteria to grow. The needle is removed from the person's arm and the vial is sent to the laboratory. A bandage is put on the puncture site to stop any bleeding.
In the laboratory, the vial is watched to see if bacteria grow. It takes from 24 to 72 hours or longer for aerobic bacteria to grow. If bacteria grow, the laboratory 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? Normal blood does not 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 their blood, a condition known as sepsis. Bacteria may also have been introduced into the blood through an open wound, by 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 can be 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 needed. If a person has bacteria in their 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 of bacteria.
Examples of aerobic bacteria that may cause blood infections include:
Staphylococcus, or staph
certain Neisseria
Pseudomonas aeruginosa
In some cases, a person may have a blood infection but the test is still negative. Sometimes, bacteria are only in the blood from time to time, which may make the test falsely negative. Repeated blood culture tests may be ordered if this type of situation is suspected.
Author: Francesca Coltrera, BA Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 6/06/2005 Contributors Potential conflict of interest information for reviewers available on request
