Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Blood Culture


At a Glance

Why Get Tested?

To check for the presence of a systemic infection; to detect and identify bacteria or yeast in the blood

When to Get Tested?

When you have signs or symptoms of sepsis, which may include fever, chills, fatigue, rapid breathing and/or heart rate, and an elevated white blood cell count

Sample Required?

Two or more blood samples drawn from separate venipuncture sites, typically from different veins in your arms

Test Preparation Needed?

None

The Test Sample

What is being tested?

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 microorganisms 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 microorganism causing the infection is identified and susceptibility testing is performed to inform the health 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 health practitioner to direct antimicrobial therapy to the specific microorganism 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 microorganism causing the infection is identified.

How is the sample collected for testing?

Usually, two to three blood samples are collected over a period of time and 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 microorganisms. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different site, usually immediately after the first venipuncture, depending on the procedure followed. Any subsequent samples may be collected at later intervals. A single blood culture is collected from children since they often have high numbers of bacteria present in their blood when they are infected. For infants and young children, 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?

No test preparation is needed.

The Test

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.

Other related tests that may be performed include:

  • Gram stain—a relatively quick test used to detect and identify the general type of bacteria
  • 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, 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 health 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 symptoms are present. 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.

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 microorganism. 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. Health 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 health practitioner 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 microorganisms are more difficult to grow in culture, and additional blood cultures using special nutrient media may be done to try to grow and identify the pathogen.
  • Viruses cannot be detected using blood culture bottles designed to grow bacteria. If the health practitioner suspects that a viral infection may be 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 health 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 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.

Rapid tests are available that can detect several different types of bacteria that are commonly known to cause infections of the blood. These tests are used in follow up to positive blood cultures to quickly identify the bacteria that are present. They can identify types such as(MRSA)Staphylcoccus aureusmethicillin-resistant , 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.