Remember: the order is blood cultures THEN antibiotics. Really.
The issue with blood cultures is what are you trying to diagnose with the test
- Bacteremia from an infection: Then you want volume. You hope that the aliquot of blood will have bacteria in it. The best would be to put the entire 6 liters into a blood culture bottle but that is not practical so get two sets from any place you can. Well, almost any place. Avoid the retinal vein, for example.
Two sets of blood cultures are better than one for finding the infecting organism (PubMed). It is all about getting enough blood volume to capture the pathogen.
Endocarditis: Then you want positive blood cultures over time. Two sets at least 4 hours apart before antibiotics if the patient is stable.
Antibiotics do screw up yield, and do so rapidly: "The rate of positive blood cultures is reduced by >50% for both septic and nonseptic patients as early as the second hour of IV antibiotic treatment"(Pubmed).
Central Venous Catheter Infections: Then you want simultaneous cultures from the line and from the periphery. If the line cultures are positive 120 minutes before the distal, the line is the source.
Since it takes about two hours from bacteremia until fever, and because with the exception of endocarditis and line infections the bacteremia is intermittent, it should come as no surprise that it doesn't matter whether or not you get blood cultures around the time of the fever (PubMed).
We do get too many blood cultures. An isolated fever or leukocytosis has little chance of a positive culture in normal (but not immunoincompetent) patients; rigors are a much more useful for predicting bacteremia as is the presence of SIRS (PubMed).
It is clear: repeating blood cultures to document clearing of the blood is a waste of time for streptococci (PubMed) and gram-negative rods (PubMed). And do not repeat the BC once antibiotics started for urosepsis; it too is a waste of time (PubMed). S. aureus and Candida are the only two bacteria that require repeat cultures on therapy.
I often think the world would be a better place if more people, upon awakening in the morning, would ask themselves the simple question, "How can I make Mark Crislip's life better?" And the answer turns out to be remarkably simple. When called for positive blood cultures, before you start antibiotics, repeat the damn blood cultures. Really, how hard can that be?
The volume of blood is impotant. Want 8–10 mL of blood/bottle, but in one study the average fill volume was 2.3 mL (Pubmed) Getting the proper ammount of blood in the culture bottles increases yield from 7 to 20%.
Last Update: 12/16/20.