Blood Culture
lso known as: Blood culture, bacterial, fungal and/or AFB
Formal name: Culture, blood
Related tests: CBC, Urine Culture, Bacterial Wound Culture, Gram Stain, CSF Analysis, Fungal Tests, Susceptibility Testing
How is it used?
Blood cultures are used to detect the presence of bacteria or yeasts in the blood, to identify the microorganism(s) present, and to guide treatment. Two or more blood cultures are typically ordered and collected as consecutive samples. 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 doctor may order blood cultures when a person is having symptoms of sepsis, which indicates that bacteria, yeast, or their toxic by-products are causing harm in the body. A person with sepsis may have:
When a person has had a recent infection, surgical procedure, prosthetic heart valve replacement, or immunosuppressive therapy, he 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.
- Chills, fever
- Nausea
- Rapid breathing, rapid heartbeat
- Confusion
- Decreased urine output
When a person has had a recent infection, surgical procedure, prosthetic heart valve replacement, or immunosuppressive therapy, he 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?
If blood cultures are positive, it most likely means that the tested person has a bacterial or yeast bloodstream infection that needs to be treated immediately, usually in a hospital. Sepsis can be life-threatening, especially in immunocompromised patients. Those who have a bloodstream infection often feel very ill. A doctor may have already started her patient on an intravenous broad spectrum antibiotic that is effective against a wide range of bacteria while waiting for the test results. When results become available, this treatment may be changed to be more specific for the microorganism that is detected in the blood cultures and its anticipated antibiotic susceptibilities.
A positive result could also be a false positive caused by skin contamination. If two or more blood culture sets are positive with the same bacteria or yeast, it is more likely that the microorganism found in the culture is causing the infection. If one set is positive and one set is negative, it could be an infection or contamination. The doctor will need to evaluate the person's clinical status and the type of microorganism found.
If the blood culture sets are both negative, (often reported as "no growth"), the probability that a person has sepsis caused by bacteria or yeasts 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:
A positive result could also be a false positive caused by skin contamination. If two or more blood culture sets are positive with the same bacteria or yeast, it is more likely that the microorganism found in the culture is causing the infection. If one set is positive and one set is negative, it could be an infection or contamination. The doctor will need to evaluate the person's clinical status and the type of microorganism found.
If the blood culture sets are both negative, (often reported as "no growth"), the probability that a person has sepsis caused by bacteria or yeasts 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 doctor 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 doctor suspects is causing the infection.
- Complete Blood Count (CBC) - an increased white blood cell (WBC) count may indicate infection.
- Complement - levels of C3 may be increased.
- A urine, sputum, or CSF culture may be positive, indicating a possible source of infection that may have spread to the blood.
Is there anything else I should know?
Because sepsis means the bacteria or yeasts have spread throughout the body, an affected person may experience many different symptoms of illness. The immune system is struggling to overcome the infection and produces many factors to kill the bacteria that can also make a person feel sick. Septicemia can cause a fall in blood pressure (shock), a rapid heart rate, and can decrease the blood flow to the brain, heart, and kidneys as well as alter blood clotting components, leading to disseminated intravascular coagulation, which can cause generalized bleeding. Bacteria in the blood may also spread to the joints and cause septic arthritis.
In January 2008, the US Food and Drug Administration cleared for marketing a rapid test that can detect methicillin-resistant Staphylcoccus aureus (MRSA) in blood. MRSA is typically difficult to treat and determining if MRSA is present in the blood with a rapid test can facilitate treatment.
source : http://labtestsonline.org/understanding/analytes/blood-culture/tab/test
In January 2008, the US Food and Drug Administration cleared for marketing a rapid test that can detect methicillin-resistant Staphylcoccus aureus (MRSA) in blood. MRSA is typically difficult to treat and determining if MRSA is present in the blood with a rapid test can facilitate treatment.
source : http://labtestsonline.org/understanding/analytes/blood-culture/tab/test
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