The Test Sample
What is being tested?
Blood cultures are procedures done to detect an infection in the blood and identify the cause. Infections of the bloodstream are most commonly caused by bacteria (bacteremia) but can also be caused by yeasts or other fungi (fungemia) or by a virus (viremia). Although blood can be used to test for viruses, this article focuses on the use of blood cultures to detect and identify bacteria and fungi in the blood.
A blood infection typically originates from some other specific site within the body, spreading from that site when a person has a severe infection and/or the immune system cannot confine it to its source. For example, a urinary tract infection may spread from the bladder and/or kidneys into the blood and then be carried throughout the body, infecting other organs and causing a serious and sometimes life-threatening systemic infection. The terms septicemia and sepsis are sometimes used interchangeably to describe this condition. Septicemia refers to an infection of the blood while sepsis is the body's serious, overwhelming, and sometimes life-threatening response to infection. This condition often requires prompt and aggressive treatment, usually in an intensive care unit of a hospital.
Other serious complications can result from an infection of the blood. Endocarditis, an inflammation and infection of the lining of the heart and/or of the heart valves, can result from a bloodstream infection. People who have prosthetic heart valves or prosthetic joints have a higher risk of a systemic infection following their surgery, although these infections are not common.
Anyone with a weakened immune system due to an underlying disease, such as leukemia or HIV/AIDS, or due to immunosuppressive drugs such as those given for chemotherapy is at a higher risk for blood infections as their immune system is less capable of killing the microorganisms that occasionally enter the blood. Bacteria and yeasts may also be introduced directly into the bloodstream through intravenous drug use or through intravenous catheters or surgical drains.
For blood cultures, multiple blood samples are usually collected for testing and from different veins to increase the likelihood of detecting the bacteria or fungi that may be present in small numbers and/or may enter the blood intermittently. This is also done to help ensure that any bacteria or fungi detected are the ones causing the infection and are not contaminants.
Blood cultures are incubated for several days before being reported as negative. Some types of bacteria and fungi grow more slowly than others and/or may take longer to detect if initially present in low numbers.
When a blood culture is positive, the specific microorganism causing the infection is identified and susceptibility testing is performed to inform the health practitioner which antibiotics are most likely to be effective for treatment.
In many laboratories, the blood culture testing process is automated with instruments continuously monitoring the samples for growth of bacteria or fungi. This allows for timely reporting of results and for the health practitioner to direct antimicrobial therapy to the specific microorganism present in the blood. Because treatment must be given as soon as possible in cases of sepsis, broad-spectrum antimicrobials that are effective against several types of bacteria are usually given intravenously while waiting for blood culture results. Antimicrobial therapy may be changed to a more targeted antibiotic therapy once the microorganism causing the infection is identified.
How is the sample collected for testing?
Usually, two to three blood samples are collected over a period of time and from different veins to increase the likelihood of detecting bacteria or fungi if they are present in the blood. Multiple blood samples help to differentiate true pathogens, which will be present in more than one blood culture, from skin bacteria that may contaminate one of several blood cultures during the collection process.
Blood is obtained by inserting a needle into a vein in the arm. The phlebotomist will put the blood into two culture bottles containing broth to grow microorganisms. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different site, usually immediately after the first venipuncture, depending on the procedure followed. Any subsequent samples may be collected at later intervals. A single blood culture is collected from children since they often have high numbers of bacteria present in their blood when they are infected. For infants and young children, the quantity of each blood sample will be smaller and appropriate for their body size.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.