
26/04/2025
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 and are sometimes used interchangeably to describe this condition. 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.
How is the sample collected for testing?
Usually, two blood samples are collected 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 microbes. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different site, immediately after the first venipuncture. A single blood culture may be collected from children since they often have high numbers of bacteria present in their blood when they have an infection. For infants and young children, the quantity of each blood sample will be smaller and appropriate for their body size.
Blood cultures are used to detect the presence of bacteria or fungi in the blood, to identify the type present, and to guide treatment. Testing is used to identify a blood infection (septicemia) that can lead to sepsis, a serious and life-threatening complication. Individuals with a suspected blood infection are often treated in intensive care units, so testing is often done in a hospital setting.
Although blood samples may be used to detect viruses, this article focuses on the use of blood cultures to detect and identify bacteria and fungi. Routine blood culture media cannot grow viruses and therefore cannot detect if the person tested has virus in their blood (viremia).
Other related tests that may be performed include: · Gram stain—a relatively quick test used to detect and identify the general type of bacteria present in other body sites, such as urine or sputum. A direct gram stain of blood is too insensitive to detect bacteria in the bloodstream. · Susceptibility testing—determines the drug (antimicrobial) that may be most effective in treating the infection Often, a complete blood count (CBC) is ordered along with or prior to the blood culture to determine whether the person has an increased number of white blood cells (or in some cases, a decreased number of white blood cells), indicating a potential infection. Sometimes other testing is also performed, such as a chemistry panel to evaluate the health status of a person’s organs, or a urine, sputum, or cerebrospinal fluid (CSF) culture to help identify the source of the original infection. This is especially true when a person has symptoms associated with a urinary tract infection, pneumonia, or meningitis.
When is it ordered?
A healthcare practitioner may order blood cultures when a person has signs and symptoms of sepsis, which indicates that bacteria, fungi, or their toxic by-products are causing harm in the body. A person with sepsis may have:
· Chills, fever
· Nausea
· Rapid breathing, rapid heartbeat
· Confusion
· Less frequent urination
As the infection progresses, more severe symptoms may develop, such as:
· Inflammation throughout the body
· The formation of many tiny blood clots in the smallest blood vessels
· A dangerous drop in blood pressure
· The failure of one or more organs
When a person has had a recent infection, surgical procedure, prosthetic heart valve replacement, or immunosuppressive therapy, the person is at a higher risk of a systemic infection and drawing blood cultures would be appropriate when an infection of the blood is suspected. Blood cultures are drawn more frequently in newborns and young children, who may have an infection but may not have the typical signs and symptoms of sepsis listed above.
Source: https://labtestsonline.org/tests/blood-culture
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A blood culture is a laboratory test that looks for the presence of germs in the blood. Learn how a blood culture test works and when it may be used.