Two or more blood samples drawn from separate venipuncture sites, typically from different veins in your arms
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 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 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, 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 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 is 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 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. 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.
How is it used?
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
- Rapid breathing, rapid heartbeat
- 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.
What does the test result mean?
Two or more blood cultures that are positive for the same bacteria or fungi means that the person tested likely has a blood infection with that microbe. The results typically identify the specific bacteria or fungi causing the infection.
Blood infections are serious and need to be treated immediately, usually in a hospital. Sepsis is a complication that can be life-threatening, especially in people with weakened immune systems. Healthcare practitioners who suspect sepsis may begin patients on intravenous broad spectrum antibiotics that are effective against a wide range of bacteria while waiting for the blood culture or susceptibility testing results. When results become available, the treatment may be changed to an antimicrobial agent that is more specific for the bacteria or fungi detected in the blood cultures.
If one blood culture set is positive and one set is negative, it may mean that an infection or skin contaminant is present. The healthcare practitioner, usually the physician, will consider the person's clinical status and the type of bacteria or fungi found before making a diagnosis. Also, additional testing may be warranted in this case.
Blood culture sets that are negative after several days (often reported as "no growth") mean that the probability that a person has a blood infection caused by bacteria or fungi is low. If symptoms persist, however, such as a fever that does not go away, additional testing may be required. A few reasons that symptoms may not resolve even though blood culture results are negative may include:
- Some microbes are more difficult to grow in culture, and additional blood cultures may be done to try to grow and identify the pathogen.
- Viruses cannot be detected using blood culture bottles designed to grow bacteria. If a viral infection is suspected as the cause of the person's symptoms, then other laboratory tests would need to be performed. The tests that would be ordered depend upon the person's clinical signs and the type of virus the healthcare practitioner suspects is causing the infection.
Results from other tests that may be done in conjunction with blood cultures may indicate sepsis even though blood cultures may be negative. Some of these may include:
- Complete blood count (CBC). An increased white blood cell (WBC) count (or in some cases a decreased WBC count) may indicate infection.
- Complement. Levels of C3 may be increased.
- A urine or sputum culture may be positive, indicating a possible source of infection that may have spread to the blood.
- A CSF analysis may reveal a possible source of infection.
Is there anything else I should know?
Because sepsis means that the bacteria or fungi have spread throughout the body, an affected person may experience many different symptoms of illness. As the immune system works to fight the infection, it produces many factors to kill the bacteria or fungi that can also make a person feel sick. Septicemia can cause a fall in blood pressure (shock), a rapid heart rate, and a decrease in blood flow to the brain, heart, and kidneys. It can also affect blood clotting factors, leading to disseminated intravascular coagulation (DIC), which can cause generalized bleeding. Bacteria in the blood may also spread to the joints and cause septic arthritis.
Recent innovations in blood culture testing have involved the development of testing methods that will quickly identify the microbes present once a blood culture is positive.
Rapid tests are available that can detect several different types of bacteria that are commonly known to cause infections of the blood. They can identify types such as methicillin-resistant Staphylcoccus aureus (MRSA), which is typically difficult to treat, and gram negative rods such as E. coli that live in the gastrointestinal tract. Rapid identification can facilitate treatment with appropriate antibiotics.
Medical researchers are also making progress on developing tests that will speed up the diagnosis-to-appropriate treatment time by:
- Identifying common pathogenic microbes directly from the blood (instead of or in addition to a blood culture)
- Helping to rule out sepsis by distinguishing between infection-positive inflammation and infection-negative inflammation
- More rapidly determining antibiotic susceptibilities (appropriate antibiotic treatment)
Why can blood culture results take so long?
The bacteria or fungi must grow to sufficient numbers in the nutrient media before they can be detected and identified. Usually this happens within a couple of days, but in some cases and with some microbes, it can take longer. Furthermore, some microbes are present in the blood in very small numbers. They must have sufficient time to reproduce and grow to quantities that can be detected.
My doctor ordered more blood cultures after the initial cultures were drawn. Why?
Additional blood cultures may be drawn to determine if bacteria present in the culture are persistent in the bloodstream (true pathogens). If they are not present in follow-up cultures, then bacteria from the skin may have contaminated the initial cultures. Additional blood cultures may also be drawn if you continue to have signs of sepsis but no microbe is recovered from the first cultures collected.
What is the treatment for sepsis?
Since sepsis is a serious condition, treatment is started immediately, without waiting for results of blood cultures. It may begin in the hospital emergency room (ER) and is frequently continued and monitored with the person in an intensive care unit (ICU). Broad-spectrum antibiotics are usually given intravenously (IV). Drug therapy may be changed to a more targeted therapy once the exact type of bacteria causing the sepsis is identified and antibiotic susceptibility testing is performed. IV fluids are given to help improve and stabilize blood pressure. Sometimes medications are given to constrict blood vessels and increase blood pressure. For more details, see the article on Sepsis.