A minimum of two sets (four bottles; one set = one aerobic bottle, one anaerobic bottle) should always be obtained (maximum = three sets/24 hours). The minimum volume of blood needed per bottle for adults is 10 ml. Thus, the minimum volume of blood per set is 20 ml. The minimum volume necessary for one pediatric bottle is 1 ml.

Ordering one set may lead to confusion if the culture is positive for an organism that is commonly a contaminant. For example, if one set is ordered and is positive for coagulase-negative staphylococci (CoNS), a common contaminant, it is impossible to determine if this represents contamination or infection. However, if two sets are ordered, and only one is positive for CoNS, this most likely represents contamination.

Please specify the desired sites of the blood draw (e.g., one from line, one peripherally).

Ideally, blood cultures should be drawn before the first dose of antibiotics, but antibiotics should not be withheld because of a delay in getting cultures drawn. Although it is common practice to wait 30-60 minutes between blood cultures, there is little data to support this practice.

If the patient is persistently febrile, obtain two sets of cultures per day for 48-72 hours. Do not continue drawing daily blood cultures beyond 72 hours.

If a vascular catheter is thought to be a potential site of infection, blood should be drawn from the catheter and the periphery. Site and time of phlebotomy should be noted. The differential time to positivity can help in assessing whether the catheter is the likely source.

Differential Time to Positivity (DTP)

a. A positive line culture result is obtained at least 2 hours earlier than a positive peripheral blood culture result.

b. Typically, the diagnosis of a line-associated infection can be made according to the following criteria: presence of an intravascular device, at least one positive blood culture obtained from a peripheral site, clinical manifestations of infection, and no other apparent source for bloodstream infection

PLUS one of the following:

positive semiquantitative catheter culture with the same organism as that isolated peripherally from the blood or differential time to positivity.