Blood culture

Alternative names 
Culture - blood

A blood culture 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.

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, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell 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 band 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 media 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 preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

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 origin of the infection. 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.

Johns Hopkins patient information

Last revised: December 7, 2012
by Sharon M. Smith, M.D.

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