Blood culture

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.

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.

No special preparation is needed. Wear a garment with loose sleeves that can be pushed up easily.

The needle prick is usually not painful.

A blood culture is performed when an infection of the blood (bacteremia or septicemia) is suspected because of symptoms such as fever, chills, orlow blood pressure. The blood culture will help identify the origin of the infection and this helps the doctor determine the best course of treatment.

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