Blood Culture

Blood cultures are done to detect and identify bacteria and yeasts in the blood. Infections of the bloodstream are most commonly caused by bacteria (bacteremia), but can also be caused by yeasts or other fungi or by a virus. The source of the infection is typically a specific site within the body. If a person's immune system cannot contain an infection at its source, such as the bladder or kidneys from a urinary tract infection, the infection may spread into the bloodstream and 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. Septicemia refers to an infection of the blood while sepsis is the body's serious, overwhelming, and sometimes life-threatening response to infection.

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 compromised immune system due to an underlying disease, such as leukemia or HIV/AIDS, or due to immunosuppressive agents such as those given for chemotherapy is at a higher risk 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.

Blood samples for cultures are drawn into vials that contain nutrients that will support the growth and allow the detection of microorganisms that prefer oxygen (aerobes) or that thrive in a reduced-oxygen environment (anaerobes). Multiple samples are usually collected at timed intervals or from different veins. This is done to aid in the detection of microorganisms that may be present in small numbers and/or may be released into the bloodstream intermittently. It is also done to help ensure that any microorganisms detected are the ones causing the infection and are not present just as a contaminant. Blood cultures are incubated for several days. In many laboratories, the process is automated with instruments continuously monitoring the samples for growth, which allows samples with bacteria or yeast in them to be detected more rapidly.

When a blood culture is positive, the specific microorganism causing the infection is identified and antibiotic susceptibility testing is performed to tell the doctor which antibiotics are most likely to be effective for treatment.

Usually, two to three blood samples are collected at timed intervals and/or from different veins to increase the likelihood of growing microorganisms if they are present in the blood and to differentiate true pathogens from skin bacteria that may contaminate the blood culture during the collection process.

Blood is obtained by inserting a needle into a vein in the arm. The drawing site will be thoroughly cleaned, usually with an isopropyl alcohol solution, followed by an iodine solution that is applied in a circular pattern and then allowed to dry. The phlebotomist will then draw about 20 milliliters (less than 1 oz) of blood and put it into two culture bottles containing broth to grow aerobic and anaerobic microorganisms. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different venipuncture site, usually immediately after the first venipuncture, depending on the procedure being followed. Any subsequent samples may be collected at timed intervals. Multiple samples are also collected from children, but the quantity of each blood sample will be smaller and appropriate for their body size.

No test preparation is needed.