Blood Culture

What do the test results mean?

Normally, blood does not contain disease-causing organisms. If the blood culture is "negative," no organisms have grown in the specially prepared containers. However, in some cases, a blood culture may be "negative" even when there are organisms present. Organisms may only enter the blood occasionally and may not have been present when the blood sample was taken. This is why more than one blood sample at different times may be taken. Sometimes, the organism may never be identified because it failed to survive or grow in the specially prepared containers.

A "positive" blood culture is abnormal. This means that an organism was found in the sample of blood tested. The presence of an organism often indicates an infection. In some cases, the organism that grows is called a "contaminant." This means the blood sample contains organisms that did not originate from the tested person's blood. For example, bacteria normally present on the skin could have entered through the needle when the sample of blood was taken. In this case, an infection is not actually present in the person's blood.

If an infection of the blood is truly present and shows up in the culture, valuable information is gained. Every organism is different and responds to different treatments. Each positive blood culture is tested to see which medicines will be effective to kill the organism. One organism may respond to a certain antibiotic while another organism may not.