If infective endocarditis is an endovasular infection with sustained and persistent bacteremia, why then we repeat the BC so often compared to other infection which perhaps has transient bacteremia
in case of bacterial endocarditis, small vegetations are formed on surface of heart valves. From there, bacteria are released intermittently. Hence the practice of more than one set of blood culture, and also from different sites.
Endocarditis may be caused by an intracellular fastidious slow-growing bacterium like Brucella. This may necessitate repeated blood culture.
I agree with brucella and other fastidious organisms whereby one need to even do a terminal culture for them and even the need to do serological testing for some like in the case of Bartonella or Q fever.. But how about the Strep and Staph?
since they are more likely to be grown easily
I wonder whether in Dukes criteria, the order of more than 2 blood culture sets is to fulfil the criteria of increased frequency of + culture (i.e. 2 or more sets + for the commonly IE assc organisms) or merely to increase the amount of blood sent since the criteria says: one need to recover at least 2 or more sets rather than one in 3
Thus I have a feeling even Dukes wanna demonstrated increased frequency thus persistent bacteremia, no?
You have to remember that though the bacteremia is sustained in endocarditis, the concentration of bacteria in the blood stream infections is usually very low. It is intuitive that the more sets, the greater the opportunity to culture bacteria in the blood stream. However, 3 sets in 24 hours ensures maximal sensitivity and there is no advantage of submitting a fourth set from a patient within 24 hours period. However, remember that the amount of blood introduced in a BC bottle is extremely important as every 1 ml increases the sensitivity by 3% up to 10 ml. (doing the math 1 set= 30% sensitivity, 2= 60% and 3=90% , anymore sets submitted would just marginally increase the sensitivity, hence advised to take 3 sets )
Also the second set will hep the laboratory and the clinician to predict the relative importance of blood culture isolates. For eg. If coagulase negative staphylococcus or S. viridans is isolated from one bottle of the set then it could mean contamination. However, if you isolate the above from all the bottles, it is clinically relevant
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