What is being tested?
Blood cultures are done to detect and identify bacteria and yeasts (a type of fungus) in the blood. Some bacteria prefer oxygen (aerobes), while others thrive in a reduced oxygen environment (anaerobes). Blood cultures are usually drawn into vials containing two types of media to detect both types of bacteria. If your blood culture is positive, the specific bacteria causing the infection will be identified and antibiotic susceptibility testing will be done to tell your doctor which antibiotics will be effective for treatment. If yeasts are causing the infection, antimicrobial therapy will be given that is appropriate for fungal infections.

Infections of the bloodstream are caused most commonly by bacteria (bacteremia), but can also be caused by a fungus (fungemia) or a virus (viremia). If your immune defenses cannot contain an infection at its source, for example in the bladder for a urinary tract infection, the infection may spread to your bloodstream and be carried throughout your body. 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 infection following their surgery, although these infections are not common. Bacteria and yeasts may be introduced directly into the bloodstream by intravenous drug us, intravenous catheters, or surgical drains. Anyone with a compromised immune system due to underlying disease (for example, leukemia) or chemotherapy (for example, immunosuppressive agents) has a higher incidence of bloodstream infections since their immune system cannot kill the microorganisms that occasionally enter the bloodstream.

How is the sample collected for testing?
Multiple samples are collected and different veins are used. This is for two reasons: 1) when multiple samples are done, you have a better chance of detecting the infection; and 2) sometimes, despite disinfection of the skin where the blood is collected, you can grow a skin contaminant, which will result in a positive blood culture that is not clinically significant (a false positive). With multiple samples, you have a better chance of ruling out a false positive and deciding what is a true bacteremia. These multiple samples pose no additional risk to you.

Blood is obtained by inserting a needle into a vein in your arm. The drawing site will be thoroughly cleaned, usually with an isopropyl alcohol solution, followed by an iodine solution that is put on in a circular pattern and then allowed to dry. The phlebotomist will then draw about 20 mls 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.