When you have signs or symptoms of sepsis like fever, chills, feeling sick, confusion and tiredness which may develop during another illness, such as a urinary tract infection (UTI), pneumonia or a skin infection
Two or more blood samples taken from separate sites (commonly from veins in your arms) into special bottles which contain a solution to help bacteria and yeast cells grow
No test preparation is needed
Blood cultures are performed to detect and identify bacteria and yeasts (a type of fungus) in the blood. Infections of the bloodstream are caused most commonly by bacteria (bacteraemia), but can also be caused by a fungus (fungaemia) or a virus (viraemia). The source of the infection is typically a specific site within the body. If the immune defences and white blood cells cannot keep the infection localised at its source it may spread to the bloodstream. When the body has an excessive response to this, causing damage to the organs of the body and stopping them functioning properly, this is known as sepsis. If the sepsis also causes reduced blood flow through the organs of the body, this is referred to as septic shock, which is a serious, overwhelming and often fatal illness.
Blood cultures are also important in the diagnosis of several other conditions. Endocarditis, an inflammation and infection of the lining of the heart and/or the heart valves, can result from a bloodstream infection. People who have artificial heart valves or artificial joints have a higher risk of infection following surgery, although these infections are not common. The direct contamination of the blood from “dirty needles” with intravenous drug use, or potentially from intravenous catheters or surgical drains can lead to bloodstream infections. Similarly, anyone with an immune system which is not working properly due to underlying disease (for example, leukaemia or HIV/AIDS) or drug therapy (for example, immunosuppressive agents) has a higher chance of bloodstream infections.
If your blood culture is positive, the specific bacteria causing the infection will be identified and antibiotic susceptibility testing will be done to tell your doctor which antibiotics will be effective for treatment. If yeasts are causing the infection, treatment will be given that is appropriate for fungal infections.
How is the sample collected for testing?
Blood is obtained by inserting a needle into a vein in the arm. The skin is thoroughly cleaned, usually with an alcohol solution that is allowed to dry. The person collecting the blood (phlebotomist) then takes a small volume (approximately 10 mL per bottle for adults or smaller volumes for babies and young children) of blood and puts it into a set of two culture bottles. One contains nutrients that will support the growth and allow the detection of microorganisms that prefer oxygen (aerobes) and the other contains nutrients for microorganisms that thrive in a reduced-oxygen environment (anaerobes). Two sets are usually collected from different veins, or through existing venous catheters, and sometimes further sets are collected at timed intervals. Microorganisms can be present in small numbers or are released into the bloodstream intermittently so the more blood that is sampled, the better the chance of detecting the infecting bacteria or yeasts. Taking multiple samples also helps to ensure that any microorganisms detected are the ones causing the infection and are not present just as contaminants from the skin. Several samples are also collected from children, but 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?
The person collecting the blood should ensure the skin at the collection site is disinfected before taking the sample to reduce the chance of the blood culture getting contaminated with bacteria that grow on the skin.
How is it used?
Blood cultures are used to detect the presence of bacteria or yeasts in the blood which may have spread from another site in the body, to identify the microorganisms present and to guide treatment. Cultures are incubated in a warm environment for several days to allow any microorganisms to multiply. In most laboratories automated instruments continuously monitor the samples for growth, allowing those with bacteria or yeasts in them to be detected more rapidly.
When a blood culture is positive, the specific microorganism causing the infection is identified and antibiotic susceptibility testing is performed to tell the doctor which antibiotics are most likely to be effective for treatment.
When is it requested?
A doctor or healthcare professional may request blood cultures if a person has symptoms of sepsis. Someone with sepsis may have:
- Chills, fever
- Rapid breathing
- Rapid heartbeat
- Decreased urine output
There is a higher risk of these symptoms following a recent infection, a surgical procedure, an artificial heart valve replacement or immunosuppressive therapy. Blood cultures are taken more frequently in newborns and young children who may have an infection but may not have the typical signs and symptoms of sepsis.
More severe infections may involve inflammation throughout the body and the formation of many tiny blood clots in the smallest blood vessels, giving rise to additional symptoms. One or more organs may be damaged and there may be a dangerous drop in blood pressure.
What does the test result mean?
If the blood culture is positive, it may mean that there is a bacterial or fungal infection in the bloodstream that needs to be treated immediately. Sepsis can be life- threatening, especially in patients whose immune system is not working properly. The doctor may start treatment with a broad-spectrum antibiotic, often given intravenously, while waiting for the test results and will adjust the treatment depending on the antibiotic susceptibility results.
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, it is more likely that the bacteria found in the culture are causing the infection. If one set is positive and one set is negative, it could be either an infection or contamination. The doctor will need to evaluate the clinical state of the patient and the type of bacteria found.
If both blood culture sets are negative, the probability of sepsis caused by bacteria or yeasts is low. However, if symptoms persist, for example a fever that does not go away, additional tests may be required. Reasons that symptoms may not resolve even though blood culture results are negative include:
- Some microorganisms are difficult to grow in culture. 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 would depend on the clinical signs and the type of virus the doctor suspects is causing the infection.
Results from other tests that may be done in conjunction with blood cultures can indicate sepsis even though blood cultures are negative. These include:
- Full Blood Count: An increased white blood cell (WBC) count may indicate infection
- Complement: Levels of C3 may be increased
- A urine, sputum, CSF or wound culture may be positive, indicating a possible source of infection that may have spread to the blood
- C-reactive protein and procalcitonin concentrations can increase in response to infection
Is there anything else I should know?
Symptoms of sepsis such as fever, chills, muscle pains, and exhaustion may also be seen with influenza (the flu). If you are ill during the flu season, your doctor may do an influenza test to rule out this viral respiratory infection. Both the flu and sepsis can be especially serious in the very young, elderly, and patients with poor immune systems (immunocompromised). It is important to tell the difference between bacteraemia/fungaemia and viraemia because while they both need to be treated promptly, the treatments are different (antibacterial or antifungal versus antiviral).
Why do I have to take antibiotics for so long if I feel better already?
It is important to remove all of the bacteria that are causing the problem. For some infections several weeks of treatment are necessary. This is especially important if you have endocarditis, which requires long-term therapy to cure.
Why do the blood culture results take so long?
The bacteria or yeast must grow 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 microorganisms it can take longer. Sometimes the microorganisms are present in the blood in very small numbers and they must have a sufficient time to grow to the quantities that can be detected. Once they have grown, the laboratory staff perform further tests to identify the bacteria and test for susceptibility to antibiotics. These tests are important to ensure the correct treatment is given, but they can take a further day or two to complete.
Why did my doctor have more blood cultures taken after the initial cultures were collected?
Additional blood cultures may be taken to find out if bacteria present in the first cultures remain in the blood stream (true pathogens). If bacteria are not present in follow up cultures, then bacteria from the skin may have contaminated the initial cultures. Additional blood cultures may also be taken if you continue to have signs of sepsis, but no microorganisms are recovered from the first cultures collected.
On This Site
Tests: Bacterial wound culture, CSF analysis, Full blood count, Fungal infections, Susceptibility testing, Influenza tests, Urine culture
Conditions:Fungal infections, Lung diseases, Meningitis, Septic arthritis, Urinary tract infection, Wound and skin infections, Sepsis