A blood culture is a test to detect germs such as bacteria or fungi in the blood. One may be ordered when a child has symptoms of an infection — such as a high fever or chills — and the doctor suspects bacteria or fungi have spread into the blood. The culture can disclose what type of germ is causing the infection, which will determine how it is treated.

To do the test, the doctor will take a blood sample and send it to a lab for testing. Results are ready in a few days, but if the child is severely ill, the doctor may start treatment before the results are complete. Treatment will be based on the most likely cause of the infection, but can be changed to be specific for the microbe found when the culture is completed and the antibiotic sensitivity of the bacteria or fungi has been determined.

Why Do a Blood Culture?

During some illnesses, certain infection-causing bacteria and fungi can invade the bloodstream and spread into other parts of the body, away from the original infection site. Their presence in the blood usually means that a child has a serious infection. Such infections usually cause a more rapid heart rate and high fever with an increase in the white blood cell count.

A blood culture can reveal a number of infections or problems, such as endocarditis, a severe and potentially life-threatening problem that occurs when bacteria in the bloodstream stick to the heart valves. A blood culture might also detect meningitis, an infection of the outer lining of the brain, osteomyelitis, a bone infection often caused by Staphylococcus aureus, and cellulitis, a skin infection that involves areas of tissue just below the skin's surface.

How Is a Blood Culture Done?

The blood culture is done with a simple blood draw performed after the skin is wiped with an alcohol pad, then smeared with a special antibacterial solution. This careful skin sterilization is important because it prevents contamination of the blood that's being drawn. It kills bacteria that may be on the surface of the skin so that they don't appear in the blood culture and interfere with identification of the germ causing the infection.

Sometimes it seems like a lot of blood is drawn for the test, but it's important that enough blood be drawn for the culture to be accurate. This may be less than a teaspoon (5 milliliters) in babies and 1 to 2 teaspoons (5 to 10 milliliters) in older children, depending on their size. The amount of blood drawn is tiny compared with the amount of blood in the body, and it's quickly replenished — usually within 24 to 48 hours.

Reviewed by: Cheryl M. Coffin, MD
Date reviewed: June 2006
Originally reviewed by: Frederick A. Meier, MD