Blood culture

Definition

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.

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 because of symptoms such as fever, chills, or low blood pressure. The blood culture will help identify the origin of the infection and this helps the doctor determine the best course of 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 sometimes comes and goes, so a series of three 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.

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