Estimation of missed bloodstream infections without the third blood culture set: a retrospective observational single-centre study

Objectives: The question of whether to obtain two or three sets of blood cultures to assess the aetiology of bloodstream infections (BSIs) remains open to debate. Few studies have assessed the proportion of BSIs missed without the third blood culture set (BCS). The aim of our study was to determine the proportion of BSIs that would be missed without the third BCS in a hospital where obtaining three BCSs is the standard of care.

Methods: We performed a descriptive retrospective study in Hospital Gregorio Marañón (Madrid) from 2010 to 2013. We included all episodes of BSI in which three BCSs were systematically obtained.

Results: We included 4000 episodes of BSI between 2010 and 2013. Without the third BCS, we would have missed 298 (7.5%) episodes of BSI: 141 (47.3%) by gram-positive microorganisms, 147 (49.3%) by gram-negative microorganisms, and 10 (3.4%) by yeasts. In 132/298 (44.3%) of the episodes another clinical sample was obtained within a week of the BCS extraction; in 101/298 (33.9%), the same microorganism was present in a significant clinical sample other than blood.

Conclusions: Our data suggest that performing a third BCS is useful, as not doing this could result in an unacceptable number of BSIs going undetected.