I have had a lot of questions about blood cultures. How many set? How many minutes apart? And so on. I put together this post to settle some of the questions.
Blood cultures are drawn to detect bacteremia. The current systems are very sensitive at detection. In fact at times too sensitive often making false positives if the site of collection was not adequately prepped. This is why two sets of blood cultures should be drawn from two separate sticks in any patient in whom it is indicated. By having two sets they can be compared to determine if a positive is merely a contaminant or real. This can affect real world treatment decisions.
Should single sets be ever drawn?
Probably not. If there are no reasons to draw two sets then there probably no good reasons to draw one either. Often time I will hear of a patient presenting with chest pain get a single set drawn because they were drawing blood anyways. This is wrong practice that is prone to give misleading information than to actually help.
Are three sets ever necessary?
Probably not. Current systems do not need more than 2 sets in a 24 hour period. The exception may be if one is trying to detect line sepsis. Then drawing off each port may be helpful.
Should blood cultures be drawn at different times?
The old practice was often to draw sets 30 minutes apart. This is no longer necessary. They should be drawn together.
Fungal blood cultures?
No longer need to be drawn in separate bottles. Standard bottles are sufficient.
What about lines?
If line sepsis is to be excluded then try to draw a culture off of each port along with a peripheral draw. Ports that turn positive more than two hours before peripheral draws are suspicious for line sepsis.
How to reduce false positives
Properly prep the site by allowing the antiseptic to dry before puncturing the vein. Any port draw should be properly prepped before drawing blood by vigorously scrubbing with antiseptic.
Blood cultures are can be invaluable in identifying bacteremia but improperly collected specimens can cause unnecessary delay in treatment.