Positive Predictive Value of True Bacteremia according to the Number of Positive Culture Sets in Adult Patients.

Bacteremia is often fatal and requires early diagnosis and treatment with adequate antimicrobial agents. 1, 2 Blood culture is regarded as the gold standard for the diagnosis of bacteremia.3 However, accurately judging the clinical significance of culture results is challenging because of possible culture contamination. Therefore, methods to help differentiate true bacteremia from contamination are required. The identification of isolated bacteria from positive blood cultures is a helpful clue for estimating the probability of true bacteremia. This probability, which is often expressed as the positive predictive value (PPV), is reported to vary by microorganism. For example, when Streptococcus pneumoniae, Escherichia coli, other members of Enterobacteriaceae, Pseudomonas aeruginosa and Candida albicans are isolated from blood culture, more than 90% are reported to be true bacteremia or fungemia. 4–7 On the other hand, when coagulase-negative Staphylococci (CNS), Corynebacterium species and Bacillus species are isolated, the probability of true bacteremia is reported to be low.4– 10 However, most of these studies were performed in the 70s through the 90s. Therefore, updated data in current patient populations and treatment methods are needed, because the ratio and causative bacteria of bloodstream infections might have changed over time as a result of the increasing use of intravascular devices and number of patients receiving immunomodulatory treatment. 11, 12 Another method for distinguishing true bacteremia from contamination is using the number of blood culture sets (each set of blood culture consists of an aerobic and anaerobic bottle containing different nutrients) that grow bacteria from among the performed culture sets. Opportunities to adopt this methodology are increasing, because the recently approved Clinical and Laboratory Standards Institute (CLSI) guideline recommends more than 2 sets of blood culture should be performed simultaneously (within minutes of one another) in patients suspected blood stream infection.13 However, it is often difficult to interpret the results when only 1 of the 2 blood culture sets is positive. In such cases, the results should be interpreted cautiously according to the identity of the isolated bacteria. In accordance with this strategy, the PPV of positive blood culture according to the number of positive blood culture sets has been examined for CNS. 7, 14 However, data about other microorganisms are rare. Therefore, this study aimed to provide information to facilitate the interpretation of positive blood cultures by analyzing the clinical significance of isolated microorganisms, the number of positive blood culture sets from among the performed sets, and clinical background characteristics of patients in current clinical settings.