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Blood Culture

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • If you have taken antibiotics recently. These medicines may stop the growth of bacteria in the culture.
  • If the blood sample is contaminated by bacteria or fungus on the skin.
  • If the blood test misses the time when bacteria actually are in the blood. Blood culture tests are done at several different times to make sure bacteria are not missed.
  • If the blood test is not done correctly or the blood sample is not processed properly. In these cases, a false-positive or false-negative result could occur.

What To Think About

  • Some types of bacteria infect the blood when another infection of the kidneys, throat, lungs, or another part of the body is present. This may not mean a serious infection of the blood.
  • About 5% of blood cultures are contaminated with normal skin bacteria (a type of staph bacteria). So it is sometimes hard to see whether the bacteria that grow in the culture are the cause of the blood infection or not. This is why more than one blood sample is taken. When the same bacteria grow in several blood cultures, it is likely that those bacteria are in the blood and are causing the infection. When staph bacteria grow in the culture in less than 48 hours, it is likely that the staph bacteria are in the blood and are causing the infection.
  • A culture that does not grow any bacteria does not always mean a blood infection is not present. The amount of blood taken, the timing of the blood sample, the type of culture done, and recent use of antibiotics can affect the growth of bacteria in the culture.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerJoseph O'Donnell, MD - Hematology, Oncology
Last RevisedAugust 6, 2012
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WebMD Medical Reference from Healthwise

Last Updated: August 06, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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