Studies have shown that, for adults, the detection of pathogenic organisms in blood is directly related to the volume of blood drawn. A false-negative blood culture result may occur if an insufficient volume of blood is added to the culture bottle.
For non-automated blood culture systems, at least 20mL of blood should be collected per culture set for an adult (a set includes 2 bottles/tubes). Automated blood culture systems may require less blood, but the volume collected should be at least the recommended minimum. Blood cultures for adult patients usually involve the collection of paired aerobic/anaerobic vials. If the volume of blood collected is less than the recommended minimum to inoculate both vials, inoculate the aerobic vial first. Any remaining blood can then be added to the anaerobic vial. It is more important that the aerobic vial contain a sufficient volume of blood than the anaerobic. Most organisms that cause bacteremia, and most pathogenic yeast are aerobic and will be recovered from the aerobic vial. Two separate venipunctures must be performed if two culture sets are ordered. Each culture bottle should be labeled with the site of specimen collection. Two culture sets increase the probability of recovering a pathogenic microorganism. Collecting from two separate venipunctures minimizes the potential for contamination. Hopefully, if one culture set was contaminated at the time of collection, the other set would be free of contaminants.
