What is Blood Culture & Sensitivity?

Also Known As: Blood Culture (bacterial fungal and/or AFB)

Formal Name: Culture blood

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 the 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 septicemia and sepsis are sometimes used interchangeably to describe this condition. Septicemia refers to an infection of the blood while sepsis is the body’s serious, overwhelming, and sometimes a life-threatening response to infection. This condition often requires prompt and aggressive treatment, usually in an intensive care unit of a hospital.

Other serious complications can result from an infection of the blood. Endocarditis, 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 weakened immune system due to an underlying disease, such as leukemia or HIV/AIDS, or due to immunosuppressive drugs such as those given for chemotherapy are at a higher risk for blood infections as their immune system is less capable of killing the microbes 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.

For blood cultures, multiple blood samples are usually collected for testing and from different veins to increase the likelihood of detecting the bacteria or fungi that may be present in small numbers and/or may enter the blood intermittently. This is also done to help ensure that any bacteria or fungi detected are the ones causing the infection and are not contaminants.

Blood cultures are incubated for several days before being reported as negative. Some types of bacteria and fungi grow more slowly than others and/or may take longer to detect if initially present in low numbers.

When a blood culture is positive, the specific microbe causing the infection is identified and susceptibility testing is performed to inform the healthcare practitioner which antibiotics are most likely to be effective for treatment.

In many laboratories, the blood culture testing process is automated with instruments continuously monitoring the samples for the growth of bacteria or fungi. This allows for timely reporting of results and for the healthcare practitioner to direct antimicrobial therapy to the specific microbe present in the blood. Because treatment must be given as soon as possible in cases of sepsis, broad-spectrum antimicrobials that are effective against several types of bacteria are usually given intravenously while waiting for blood culture results. Antimicrobial therapy may be changed to a more targeted antibiotic therapy once the microbe causing the infection is identified.

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. The 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.

How is it used?

Blood cultures are used to detect the presence of bacteria or fungi in the blood, identify the type present, and 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 a 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 another 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.