A blood culture is a laboratory test to check for bacteria or other microorganisms in a blood sample. Most cultures check for bacteria.
A culture may be done using a sample of blood, tissue, stool, urine, or other fluid from the body. See also:
Culture - blood
How the test is performed
A blood sample is needed. For information on how this is done, see: Venipuncture
The sample is sent to a laboratory, where it is placed in a special dish and watched to see if germs grow. This is called a culture. Most cultures check for bacteria. If bacteria does grow, further tests will be done to identify the specific type.
A gram stain may also be done. A gram stain is a method of identifying microorganisms (bacteria) using a special series of stains (colors). For example, see skin lesion gram stain
Why the test is performed
Your doctor may order this test if you have symptoms of a serious blood infection. Symptoms include high fever, chills, rapid breathing and heart rate, confusion, and very low blood pressure. See also: bacteremia
The blood culture will help identify the type of bacteria causing the infection. This helps the doctor determine your best course of treatment.
A normal value means that no bacteria or other germs were seen in your blood sample.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
An abnormal (positive) result usually means that you have an infection called sepsis. Sepsis is a severe illness in which your blood is overwhelmed by bacteria. If you have sepsis, you will be admitted to a hospital.
Other types of germs, such as a fungus or a virus, may also be found in a blood culture.
Sometimes, an abnormal result can be due to contamination. This means bacteria may be found, but it came from your skin or from the lab equipment, instead of your blood. This is called a false-positive result. It means you do not have a true infection.
What the risks are
The blood culture is done in a lab. There are no risks to the patient. For information on risks related to giving a blood sample, see venipuncture.
A bacterial blood infection sometimes comes and goes, so a series of three blood cultures may be done to confirm results.
Shapiro NI, Zimmer GD, Barkin AZ. Sepsis syndromes. In: Marx, JA, ed. Rosen's Emergency Medicine: Concepts and Clinical Practice. 6th ed. Philadelphia, Pa: Mosby Elsevier; 2006: chap 136.
Croft AC, Woods GL. Specimen collection and handling for diagnosis of infectious diseases. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2006:chap 63.
Murray PR, Witebsky FG. The clinician and the microbiology laboratory. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 17.
Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.