At a Glance
Why Get Tested?
When to Get Tested?
Two or more blood samples drawn at timed intervals or from separate venipuncture sites, typically from different veins in your arms
Test Preparation Needed?
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.
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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
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.
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://220.127.116.11/presenter.jhtml?identifier=9500 through http://18.104.22.168.
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.