Blood Culture Test

What is a blood culture test?

A blood culture is a test done to see if a bacterial or fungal infection is present in your child’s bloodstream. The blood culture allows your child’s healthcare provider to identify the specific bacteria or fungus causing the infection, and then decide what antibiotics can be used to treat the germ. Because an infection in the blood can be very serious, it is important to know what kind of bacteria or fungus is causing it and what medicine will best treat the infection.

Why is this test done?

Usually this test is done when your child has signs of an infection and your child’s provider suspects the infection is in your child’s blood. Some infections start in the blood, particularly for children who have some sort of a catheter in place, but most infections start somewhere else in the body and then spread to the blood

Babies cannot tell us where things hurt. Their immune systems are not fully developed, and they have not yet gotten all childhood vaccinations. This is why it is more common for younger children to have a blood culture test done. Babies younger than 4 to 8 weeks with a temperature between 100.4 to 101 F (38.0 to 38.3 C) will often have blood culture tests.

How do I prepare my child for this test?

Make sure your child’s healthcare provider knows about any medicines (especially antibiotics), herbs, or supplements that your child is taking. It is important to do this blood test before any antibiotics are given to your child. Be sure to tell the doctor and person who draws the blood if your child has an allergy to iodine because this is often used to clean the skin before a blood culture. Don't stop any of your child’s regular medicines without first talking to your child’s provider about it.

How is the test done?

The skin on your child’s arm is carefully cleaned and then a small amount of blood is taken from a vein in your child’s arm with a needle. A simple fingerstick cannot be done because even if the skin has been cleaned, germs can get into the sample and give false results. Blood may be taken from more than 1 site. The blood is collected in tubes and sent to a lab.

Having this test will take a few minutes. The blood is drawn very carefully so that no germs from your child’s skin or the lab techs hands get in the sample. There is no risk of getting AIDS, hepatitis, or any other blood-borne disease from this test.

If bacteria are present in the blood, they usually grow within 24 to 36 hours. Fungus takes longer to grow.

If a germ does grow, the lab will narrow down what type of germ it is. Over the next 24 to 48 hours the lab then grows the germ on special plates and finds out what medicines will work best to kill the germ.

What does the test result mean?

A blood culture test is positive if a bacteria or fungus grows in the culture. It is negative if nothing grows. If the test is negative, it may mean:

• Your child does not have an infection.
• Your child has an infection caused by a virus, rather than by bacteria or fungus.
• The culture did not grow properly. This may be because your child took an antibiotic, and the germ did not grow in the test tube, even though it’s in your child’s body. It may be because the blood was contaminated by something on your child’s skin.

Your child’s healthcare provider may have the test done again in several hours or days.

When the test is positive, it may mean:

• The bacteria or fungus grown in the blood culture are from an infection in your child’s blood.
• The bacteria or fungus found in the blood culture are from your child’s skin and were not actually in your child’s blood.

What if my child’s test result is not normal?

Test results are only one part of a larger picture that takes into account your child’s medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your child’s healthcare provider about your child’s result and ask questions. If your child’s test results are not normal, ask your child’s healthcare provider:

• if your child needs more tests
• what kind of treatment your child might need
• when your child needs to be tested again.