Testing Information

What is a Blood Culture?

A microbe is a general term that refers to tiny organisms, such as bacteria, viruses, fungi, and parasites. Many of these microbes are a normal part of our environment. In fact, we couldn't survive very well in a world completely free of them. Microbes, including the bacteria and fungi that cause serious infections, live on us, inside us, and all around us.

The body has its own flora of microbes that live on the surface of the skin and on the linings of the mouth and intestines. Normally, the tissues that line these surfaces are colonized by a variety of bacteria and fungi. These microscopic organisms usually are denied entry to the deeper tissues of the body, and bacteria that find their way into normally sterile places, like the blood, usually are destroyed quickly by white blood cells and other parts of the immune system.

During certain illnesses, surface-colonizing microbes or germs causing infection somewhere in the body can leak into the bloodstream. Microbes in the bloodstream sometimes can spread to other parts of the body - after all, the blood travels just about everywhere. The bacteria or fungi can then spread to other areas away from the original infection site. One example of this is when bacteria in the bloodstream stick to the heart valves, causing a condition known as endocarditis, a severe and potentially life-threatening problem.

The presence of bacteria or fungi in the blood usually means that the patient has a serious infection. Such infections usually cause a high fever with an increase in the white cell count in the blood. People who have serious infections usually look very sick. Physicians describe illness involving a spreading of bacteria through the blood as sepsis. Sepsis is a cause for concern, and physicians usually start antibiotics and other appropriate therapy with little delay.

Blood Culture BottlesIf the doctor knew exactly which bacteria or fungi were causing the illness and which antibiotic would kill the germ, then it would be easy to choose the right treatment. But to get that information, the germ must be caught, grown in a culture, and tested against various antibiotics. This procedure is called C&S or Culture and Sensitivity and this process usually takes several days or longer in some cases. However when a patient is severely ill, physicians usually don't wait to start treatment. In that case, the physician decides which germs are most likely to be the cause of the infection and begins treatment, then changing the antibiotic course if needed when the results of blood culture(s) returns.

How Is a Blood Culture Done?
The test is performed after first wiping the skin with an alcohol pad. Then a special antibacterial solution is smeared onto the skin and allowed to dry. This careful skin sterilization is important to prevent contamination of the blood that's being drawn. The point is to kill all the bacteria that may be on the surface of the skin so that they don't appear in the blood culture and interfere with the identification of the germ that actually is 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.