Blood Cultures: Related Pages

At a Glance

Why Get Tested?

To check for the presence of a systemic infection

When to Get Tested?

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

Sample Required?

Two or more blood samples taken from separate sites (commonly from veins in your arms)

Test Preparation Needed?

No test preparation is needed.

The Test Sample

What is being tested?

Blood cultures are done to detect and identify bacteria and yeasts (a type of fungus) in the blood. Some bacteria prefer oxygen (aerobes), while others thrive in a reduced oxygen environment (anaerobes). Blood cultures are usually collected into two types of media to detect both types of bacteria. 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.

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, a condition called septicaemia.

If the microorganisms then infect the organs of the body and affect their function, it produces a condition called sepsis and the result is sometimes a serious, overwhelming illness, septic shock, that is often fatal. 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 a immune system which is not working proper;ly due to underlying disease (for example, leukaemia or HIV/AIDS) or drug therapy (for example, immunosuppressive agents) has a higher incidence of bloodstream 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 phlebotomist then takes a small volume 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, and sometimes further sets are collected at timed intervals. This is done to detect microorganisms that are present in small numbers or are released into the bloodstream intermittently. It is also done to help 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.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

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 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 many 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.

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When is it requested?

A doctor may request blood cultures if a person has symptoms of sepsis. Someone with sepsis may have:

Chills, fever
Nausea
Tiredness
Confusion
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.

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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

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Is there anything else I should know?

In sepsis the bacteria or yeasts have spread throughout the body and therefore many different symptoms of illness may be seen. The immune system struggles to overcome the infection and produces many factors to help kill the bacteria and it is these that can cause sickness and vomiting. Severe sepsis can result in shock with a rapid heart rate and decreased blood flow to the brain, heart, and kidneys. In addition, it can alter blood clotting components leading to disseminated intravascular coagulation (which can cause generalised bleeding). Bacteria in the blood may attach to heart valves and cause damage and heart murmurs (endocarditis). Bacteria in the blood may also spread to the joints and cause septic arthritis

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 septicaemia can be especially serious in the very young, elderly, and patients with poor immune systems (immunocompromised). It is important to tell the difference between septicaemia and viraemia because while they both need to be treated promptly, the treatments are different (antibacterial versus antiviral).

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Common Questions

1. 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.

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2. 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.

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3. 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.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.