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Blood cultures should be obtained (prior to initiation of antimicrobial therapy) for any patient in whom there is suspicion of bacteremia, including hospitalized patients and selected outpatients with fever and leukocytosis or leukopenia. However, a normal white blood count does not rule out bacteremia. Circumstances in which blood cultures are especially important include sepsis, meningitis, osteomyelitis, arthritis, endocarditis, peritonitis, pneumonia, and fever of unknown origin.
Prior to initiation of antimicrobial therapy in adults, at least two, preferably three, 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.
Important measures to reduce contamination include effective disinfection of the venipuncture site and avoiding blood culture collection through existing intravenous lines. The appropriate volume for adults is a minimum of 10 mL (and preferably 20 mL) of blood; the appropriate volumes for children are summarized in the Table．
There are two clinical patterns of bacteremia, intermittent and continuous. Intermittent bacteremia implies that bacteria are present in the blood for periods of time followed by nonbacteremic periods; this is the most common pattern. Continuous bacteremia usually reflects a persistent endovascular infection such as endocarditis.
Organisms for which it can be difficult to distinguish between pathogenicity and contamination include Propionibacterium acnes, Corynebacterium species, Bacillus species, and coagulase-negative staphylococci; the likelihood of pathogenicity is increased if the organism is observed in multiple blood cultures obtained from separate venipunctures．