Anaerobic Bacteria, Blood Culture For

Anaerobic Bacteria, Blood Culture For

Alternate Names

  • blood culture for anaerobic bacteria

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?

This test is done most often on very young or old people and people with weakened immune systems. However, this test may be done 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.
Signs of a blood infection may include:
  • fever with or without chills
  • nausea and vomiting
  • low blood pressure
  • fast heartbeat, known as tachycardia
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 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 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 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 anaerobic bacteria that may cause infections of the blood include:
  • Bacteroides
  • Anaerobic streptococci
  • Fusobacterium
  • Clostridium
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.

Sources

Anderson KN, Anderson LE, Glanze WD. Mosby's medical, nursing, and allied health dictionary, 5th ed. St. Louis, Missouri: Mosby-Year Book, Inc., 1998.

Miller BF, Keane, CB. Encyclopedia and dictionary of medicine, nursing, and allied health, 4th ed. Philadelphia, Pennsylvania: WB Saunders Company, 1987.

Pagana KD and Pagana T. Mosby's manual of diagnostic and laboratory tests, 4th ed. St. Louis: Mosby, 1998.

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