At a Glance
Why Get Tested?
When to Get Tested?
Two or more blood samples drawn from separate venipuncture sites, typically from different veins in your arms
Test Preparation Needed?
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.
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.
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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
Lin, J. (Updated 20012 February 13). Blood Culture. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/2093349-overview through http://emedicine.medscape.com. Accessed August 2013.
Hazen, K. (Revised 2013 February). Culture. Merck Manual for Healthcare Professionals [On-line information]. Available online through http://www.merckmanuals.com. Accessed August 2013.
Fisher, M. et. al. (Updated 2013 March 20). Sepsis. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Sepsis.html through http://www.arupconsult.com. Accessed August 2013.
Durani, Y. (Reviewed 2012 May). Blood Culture. KidsHealth from Nemours [On-line information]. Available online at http://kidshealth.org/parent/general/sick/labtest3.html through http://kidshealth.org. Accessed August 2013.
Vorvick, L. (Updated 2011 October 26). Blood culture. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003744.htm through http://www.nlm.nih.gov. Accessed August 2013.
Flayhart, D. (2012 March). Blood cultures and detection of sepsis. Medical Laboratory Observer [On-line information]. Available online at http://www.mlo-online.com/articles/201203/blood-cultures-and-detection-of-sepsis.php through http://www.mlo-online.com. Accessed August 2013.
Andrew Hemmert, A. and Garrone, N. (2013 June) Rapid methods for pathogen detection in bloodstream infections. Medical Laboratory Observer [On-line information]. Available online at http://www.mlo-online.com/articles/201306/rapid-methods-for-pathogen-detection-in-bloodstream-infections.php through http://www.mlo-online.com. Accessed August 2013.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 166-167.
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007 Pp 1189-1191.
Forbes BA, Sahm DF, Weissfeld AS, Bailey & Scott's Diagnostic Microbiology 12th Edition: Mosby Elsevier, St. Louis, MO; 2007, Pp 778-781.
Cunha B (July 13, 2012). Bacterial Sepsis. Medscape Reference article. Avaialble online at http://emedicine.medscape.com/article/234587-overview through http://emedicine.medscape.com. Accessed September 2013.
(June 27, 2012) FDA Press Release. FDA allows marketing of first test to identify certain bacteria associated with bloodstream infections. Available online at http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm309950.htm through http://www.fda.gov. Accessed September 2013.
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://126.96.36.199/presenter.jhtml?identifier=9500 through http://188.8.131.52.
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.
Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 172-173.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1532-1535.
Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 778-797.
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.