Blood Culture

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Also known as: Blood culture, bacterial, fungal and/or AFB

Formal name: Culture, blood

At a Glance

Why Get Tested?

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

When to Get Tested?

When you have signs or symptoms of sepsis, such as fever of unknown origin, chills, fatigue, and an elevated white blood cell count

Sample Required?

Two or more blood samples drawn at timed intervals or 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 done to detect and identify bacteria and yeasts in the blood. Infections of the bloodstream are most commonly caused by bacteria (bacteremia), but can also be caused by yeasts or other fungi or by a virus. The source of the infection is typically a specific site within the body. If a person's immune system cannot contain an infection at its source, such as the bladder or kidneys from a urinary tract infection, the infection may spread into the bloodstream and 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. Septicemia refers to an infection of the blood while sepsis is the body's serious, overwhelming, and sometimes life-threatening response to infection.

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 compromised immune system due to an underlying disease, such as leukemia or HIV/AIDS, or due to immunosuppressive agents such as those given for chemotherapy is at a higher risk 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.

Blood samples for cultures are drawn into vials that contain nutrients that will support the growth and allow the detection of microorganisms that prefer oxygen (aerobes) or that thrive in a reduced-oxygen environment (anaerobes). Multiple samples are usually collected at timed intervals or from different veins. This is done to aid in the detection of microorganisms that may be present in small numbers and/or may be 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 a contaminant. Blood cultures are incubated for several days. In many laboratories, the process is automated with instruments continuously monitoring the samples for growth, which allows samples with bacteria or yeast 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.

How is the sample collected for testing?

Usually, two to three blood samples are collected at timed intervals and/or from different veins to increase the likelihood of growing microorganisms if they are present in the blood and to differentiate true pathogens from skin bacteria that may contaminate the blood culture during the collection process.

Blood is obtained by inserting a needle into a vein in the arm. The drawing site will be thoroughly cleaned, usually with an isopropyl alcohol solution, followed by an iodine solution that is applied in a circular pattern and then allowed to dry. The phlebotomist will then draw about 20 milliliters (less than 1 oz) of blood and put it into two culture bottles containing broth to grow aerobic and anaerobic microorganisms. These two bottles constitute one blood culture set. A second set of blood cultures should be collected from a different venipuncture site, usually immediately after the first venipuncture, depending on the procedure being followed. Any subsequent samples may be collected at timed intervals. Multiple 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, 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.

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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:

Chills, fever
Nausea
Rapid breathing, rapid heartbeat
Confusion
Decreased urine output

More severe symptoms may involve inflammation throughout the body and formation of many tiny blood clots in the smallest blood vessels. One or more organs may begin to stop working and there may be a dangerous drop in blood pressure.

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.

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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:

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.

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, sputum, or CSF 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?

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.

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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 eliminate 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 weeks of antibiotic 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. Furthermore, some microorganisms are present in the blood in very small numbers. They must have a sufficient time to reproduce and grow to quantities that can be detected.

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3. Why did my doctor order more blood cultures after the initial cultures were drawn?

Additional blood cultures may be drawn to determine if bacteria present in the culture are persistent in the blood stream (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 microorganism is recovered from the first cultures collected.

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Article Sources

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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.

Sources Used in Current Review

Shapiro, N. et. al. (2008 November 06). Who Needs a Blood Culture? A Prospective Derived and Validated Prediction Rule. Medscape from the Journal of Emergency Medicine [On-line information] Available online at http://www.medscape.com/viewarticle/581631 through http://www.medscape.com. Accessed May 2009.

Smith, D. S. (Updated 2007 December 03). Blood Culture. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003744.htm. Accessed May 2009.

Barenfanger, J. et. al. (2009 January 27). Decreased Mortality Associated With Prompt Gram Staining of Blood Cultures. Medscape from American Journal of Clinical Pathology [On-line information]. Available online at http://www.medscape.com/viewarticle/585709 through http://www.medscape.com. Accessed May 2009.

Cunha, B. (2008 August 1). Sepsis, Bacterial. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/234587-overview through http://emedicine.medscape.com. Accessed May 2009.

Weil, M. H. (Revised 2007 December). Sepsis and Septic Shock. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/sec06/ch068/ch068a.html through http://www.merck.com. Accessed May 2009.

Anderson-Berry, A. et. al. (Updated 2008 November 6). Neonatal Sepsis. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/978352-overview through http://emedicine.medscape.com. Accessed May 2009.

Diab, M. et. al. (2008 May 28). Rapid Identification of Methicillin-Resistant Staphylococci Bacteremia Among Intensive Care Unit Patients. Medscape Today from The Medscape Journal of Medicine [On-line information]. Available online at http://www.medscape.com/viewarticle/572647 through http://www.medscape.com. Accessed May 2009.

(January 2, 2008) US Food and Drug Administration. Press Release: FDA Clears First Quick Test For Drug-Resistant Staph Infections. Available online at http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm161552.htm through http://www.fda.gov. Accessed July 2009.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Bonow, R. (Committee Member) et. al. (1998). ACC/AHA Guidelines for the Management of Patients With Valvular Heart Disease -- IV. Evaluation and Management of Infective Endocarditis -- A. Antimicrobial Therapy. American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Management of Patients With Valvular Heart Disease) [On-line report]. Available online at http://216.185.112.5/presenter.jhtml?identifier=9500 through http://216.185.112.5.

Medical Encyclopedia. Sepsis. Drkoop.com [On-line information]. Available online at http://www.drkoop.com/conditions/ency/article/000666.htm through http://www.drkoop.com.

Bailey and Scott's Diagnostic Microbiology, 10th ed. (1998) B.A. Forbes, D.F. Sahm, and A. S. Weissfeld (ed.), Mosby, Inc. Pg. 283-304.