Lee A, et. al. Detection of bloodstream infections in adults: how many blood cultures are needed? J Clin Microbiol. 2007 Nov;45(11):3546-8.
Objective: To assess the effect of using three or more blood cultures in diagnosing bacteremia.
Conclusion: Some additional benefit of the 3rd blood culture; marginal additional benenefit of the 4th blood culture, but maybe for some patients.
Methods: Retrospective study of all blood cultures from consecutive adult patients admitted to two teaching hospitals from 2004 through 2005. Included patients with 3 or more cultures taken during 24 hour period and whose records indicated true infection rather than specimen contamination. A possible limitation of the method is that if 2 cultures taken at the same time from two sites were negative and then a third culture was taken later and was positive, the authors counted that as first positive on the third culture even though the positive culture was actually the second time a culture was taken.
Data: Table 2 (p. 3547) summarizes the number of cultures needed to detect common pathogens in unimicrobial infections. Overall, of the 629 unimicrobial episodes, the percentage of cultures that identified pathogens were:
1 culture 73%
2 cultures: 90%
3 cultures: 98%
4 Cultures: 99.7%