At a glance
Also known as
Culture blood; BC
Why get tested?
Blood cultures are used to check for the presence of a bloodstream infection
When to get tested?
When you have signs or symptoms of a bacterial infection, such as fever, chills and tiredness, which also may be accompanied by other illnesses, such as urinary tract infection (UTI), pneumonia, diarrhoea, delirium sepsis or skin infection
Sample required?
Two or more blood samples drawn from separate sites (commonly from veins in your arms). Multiple samples may be collected to increase the chance of detecting the infection.
Test preparation needed?
Skin antisepsis should be performed to minimise contamination.
What is being tested?
Blood cultures are done to detect and identify and yeasts (a type of ) in the blood. Some bacteria prefer oxygen (aerobes), while others thrive in a reduced oxygen environment (anaerobes). Blood cultures are usually drawn into 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, treatment will be given that is appropriate for fungal infections.
Infections of the bloodstream are caused most commonly by bacteria (), but can also be caused by a fungus (fungaemia) or a (viraemia). If your immune defences and white blood cells 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 and infection of the lining of the heart and/or of 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 their 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 a compromised immune system due to underlying disease (for example, leukaemia) or drug therapy (for example, agents) has a higher incidence of bloodstream 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 antiseptic agent sucha as iodine or alcohol. The blood collector or phlebotomist, then takes a small volume (approximately 10mL per bottle) 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 nutients for microorganisms that thrive in a reduced-oxygen environment (anaerobes).
Two sets are usually collected from different veins, or through existing venous cathethers, and sometimes further sets are collected at timed intervals. This is done to detect microorganisms that ar present in small numbers or are 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 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.
Multiple samples are collected and different veins are used. This is for two reasons:
- when multiple samples are done, there is a better chance of detecting the infection; and
- sometimes, despite disinfection of the skin where the blood is collected, a skin contaminant can grow, which will result in a positive blood culture that is not clinically significant (a ).
With multiple samples, there is a better chance of ruling out a false positive and deciding what is a true bacteraemia. These multiple samples pose no additional risk to you.
See sampling guideline here.
Is any test preparation needed to ensure the quality of the sample?
Skin antisepsis should be performed to minimise contamination.