Blood culture

Blood cultures are performed to detect and identify bacteria and yeasts (a type of fungus) in the blood. Infections of the bloodstream are caused most commonly by bacteria (bacteraemia), but can also be caused by a fungus (fungaemia) or a virus (viraemia). The source of the infection is typically a specific site within the body. If the immune defences and white blood cells cannot keep the infection localised at its source it may spread to the bloodstream. When the body has an excessive response to this, causing damage to the organs of the body and stopping them functioning properly, this is known as sepsis. If the sepsis also causes reduced blood flow through the organs of the body, this is referred to as septic shock, which is a serious, overwhelming and often fatal illness.

Blood cultures are also important in the diagnosis of several other conditions. Endocarditis, an inflammation and infection of the lining of the heart and/or the heart valves, can result from a bloodstream infection. People who have artificial heart valves or artificial joints have a higher risk of infection following surgery, although these infections are not common. The direct contamination of the blood from “dirty needles” with intravenous drug use, or potentially from intravenous catheters or surgical drains can lead to bloodstream infections. Similarly, anyone with an immune system which is not working properly due to underlying disease (for example, leukaemia or HIV/AIDS) or drug therapy (for example, immunosuppressive agents) has a higher chance of bloodstream infections.

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, treatment will be given that is appropriate for fungal infections.

How is the sample collected for testing?

Blood is obtained by inserting a needle into a vein in the arm. The skin is thoroughly cleaned, usually with an alcohol solution that is allowed to dry. The person collecting the blood (phlebotomist) then takes a small volume (approximately 10 mL per bottle for adults or smaller volumes for babies and young children) of blood and puts it into a set of two culture bottles. One contains nutrients that will support the growth and allow the detection of microorganisms that prefer oxygen (aerobes) and the other contains nutrients for microorganisms that thrive in a reduced-oxygen environment (anaerobes). Two sets are usually collected from different veins, or through existing venous catheters, and sometimes further sets are collected at timed intervals. Microorganisms can be present in small numbers or are released into the bloodstream intermittently so the more blood that is sampled, the better the chance of detecting the infecting bacteria or yeasts. Taking multiple samples also helps to ensure that any microorganisms detected are the ones causing the infection and are not present just as contaminants from the skin. Several samples are also collected from children, but the quantity of each blood sample will be smaller and appropriate for their body size.

Is any test preparation needed to ensure the quality of the sample?

The person collecting the blood should ensure the skin at the collection site is disinfected before taking the sample to reduce the chance of the blood culture getting contaminated with bacteria that grow on the skin.