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Blood Culture

A blood culture is a test that looks for germs such as bacteria or fungi in the blood. A doctor might order this test when a child has symptoms of an infection — such as a high fever or chills — and the doctor suspects germs have spread into the blood. The culture can show 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.

If a child is severely ill, the doctor may start treatment before the results are complete, basing treatment on the most likely cause of the infection. Once the culture results and most effective antibiotics have been identified (called antibiotic sensitivity) doctors can adjust the treatment choice. 

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, high fever, and an increase in the number of white blood cells, the cells in the body that fight infections.

A blood culture can be a sign of underlying 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.

Some other possible causes of bloodstream infections include osteomyelitis, a bone infection often caused by the bacteria Staphylococcus aureus, and cellulitis, a skin infection that involves areas of tissue just below the skin's surface.

If an underlying cause is suspected, a doctor may order additional tests to find the cause of the bloodstream infection.

How Is a Blood Culture Done?

To do a blood culture, the nurse or phlebotomist (technician who draws blood) cleans the skin with an alcohol pad and a special antibacterial solution. Then this person inserts a small sterile needle through the skin into a vein and draws the blood directly into a culture bottle. Careful skin cleaning 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 be drawn for the culture to be accurate. This may be less than a teaspoon (5 milliliters) in babies and 1–2 teaspoons (5–10 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 (within 24–48 hours).

After the blood is sent to the lab, results usually are available in 1–2 days. If you have any questions about the test, be sure to speak with your doctor.

Date reviewed: September 2015