Blood cultures for the detection of bacteremia
- Gary V Doern, MD
Gary V Doern, MD
- Emeritus Professor of Pathology
- University of Iowa College of Medicine
Blood is one of the most important specimens received by the microbiology laboratory for culture, and culture of blood is usually the most sensitive method for detection of bacteremia or fungemia. Issues related to types of bacteremia, indications, and technique for blood cultures will be reviewed here. Issues related to specimen transport and Gram stain interpretation are discussed in detail separately. (See "Microbiology specimen collection and transport" and "Approach to Gram stain and culture results in the microbiology laboratory".)
INDICATIONS FOR CULTURES
Blood cultures should be obtained (prior to initiation of antimicrobial therapy) for any patient in whom there is suspicion of bacteremia or fungemia, including hospitalized patients and selected outpatients with fever and leukocytosis or leukopenia . However, a normal white blood cell count does not rule out bacteremia [2,3]. Circumstances in which blood cultures are especially important include known or suspected sepsis, meningitis, osteomyelitis, arthritis, endocarditis, peritonitis, pneumonia, and fever of unknown origin.
Indications for follow-up blood cultures are discussed below. (See 'Follow-up blood cultures' below.)
METHOD FOR OBTAINING CULTURES
In general, adult patients with bacteremia are likely to have low quantities of bacteria in the blood, even in the setting of sepsis. In addition, bacteremia in adults is generally intermittent. For this reason, multiple blood cultures, each containing large volumes of blood, are required to detect bacteremia. Prior to initiation of antimicrobial therapy, at least two sets of blood cultures taken from separate venipuncture sites should be obtained . The technique, number of cultures, and volume of blood are more important factors for detection of bacteremia than timing of culture collection; these are discussed further in the following sections.
Technique — Careful technique is critical to avoid contamination of the blood culture media by normal skin flora during the process of collection. This is important because normal bacterial skin flora can also cause systemic disease, such as infective endocarditis, and in some circumstances blood culture contamination can make it difficult to distinguish between false-positive results and true infection. Measures to reduce contamination include effective disinfection of the venipuncture site and avoiding blood culture collection through existing intravenous lines [5-9].
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- INDICATIONS FOR CULTURES
- METHOD FOR OBTAINING CULTURES
- Number of cultures
- Volume of blood
- CULTURE MEDIA
- DURATION OF INCUBATION
- ORGANISM IDENTIFICATION
- INTERPRETING POSITIVE CULTURES
- Types of bacteremia
- FOLLOW-UP BLOOD CULTURES
- INFORMATION FOR PATIENTS
- SUMMARY AND RECOMMENDATIONS