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Blood culture
Definition:
This is a test to determine if microorganisms such as bacteria, mycobacteria, or fungus are present in the blood. A sample of blood is put in a special laboratory preparation and is incubated (in a controlled environment) for 1 to 7 days.

Alternative Names:
Culture - blood

How the test is performed:

In this test it is important that the blood sample does not become contaminated by organisms on the skin or equipment used in preparing the test. A strict sterile technique is followed to obtain and prepare the specimen.

Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

The culture is examined for the presence of microorganisms over several days. If organisms are present, further culturing may take place to identify the organisms. A Gram stain may also be done to classify the organism so that antibiotic therapy can be started before final culture results are available.

The initial sample must be placed in the correct type of laboratory media. Most cultures are for bacteria. Other medias are available for mycobacteria and fungal infections.
How to prepare for the test:
No special preparation is needed. Wear a garment with loose sleeves that can be pushed up easily.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
The needle prick is usually not painful.
Why the test is performed:
A blood culture is performed when an infection of the blood (bacteremia or septicemia) is suspected in the presence of fever, chills, low blood pressure, or other symptoms. The blood culture will help identify the infections origin. The results are used as a basis for determining appropriate antimicrobial therapy for treatment.
Normal Values:
There is no growth of microorganisms in the culture medium.
What abnormal results mean:
Positive results usually mean that infectious microorganisms are evident in the bloodstream. Sometimes it is just a contaminating bacteria, not a true infection, thus a false-positive. Your health care provider should be able to help you determine if it is a true infection or a contaminant.
What the risks are:
There may be some bruising at the site of the venipuncture. Bleeding can also occur.
Special considerations:
Bacteremia is sometimes intermittent, so a series of 3 blood cultures may be performed before a negative result is confirmed.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Review Date: 1/13/2002
Reviewed By: Camille Kotton, M.D., Infectious Diseases Division, Massachusetts General Hospital and Brigham and Women's Hospital, Boston, MA. Review provided by VeriMed Healthcare Network.
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