A blood culture is a laboratory test to check for bacteria or other microorganisms in a blood sample. Most cultures check for bacteria.
Culture - blood
How the Test is Performed
A blood sample is needed.
The site where blood will be drawn is first cleaned with an antiseptic such as chlorhexidine. This reduces the chance of an organism from the skin getting into (contaminating) the blood sample and causing a false-positive result (see below).
The sample is sent to a laboratory. There, it is placed in a special dish (culture). It is then watched to see if bacteria or other disease-causing organisms grow. A gram stain may also be done. A gram stain is a method of identifying microorganisms (bacteria) using a special series of stains (colors).
A bacterial blood infection sometimes comes and goes, so a series of three blood cultures may be done to confirm results.
How to Prepare for the Test
There is no special preparation.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
Your doctor may order this test if you have symptoms of a serious blood infection (sepsis
). Symptoms include high fever, chills, rapid breathing and heart rate, confusion, and very low blood pressure.
The blood culture helps identify the type of bacteria causing the infection. This helps the doctor determine how best to treat the infection.
A normal value means that no bacteria or other germs were seen in your blood sample.
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.
Veins and arteries vary in size from one patient to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Croft AC, Woods GL. Medical bacteriology. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 57.
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.
Munford RS, Suffredini AF. Sepsis, severe sepis, and septic shock. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2009:chap 70.
Daniel Levy, MD, PhD, Infectious Diseases, Lutherville Personal Physicians, Lutherville, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.