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Many of the variables that affect the laboratory diagnosis of bacteremia and fungemia have been addressed in this article. Whereas the scientific basis and principles for blood cultures are well-established, and the methodology has improved, the diagnosis of bacteremia and fungemia still depends greatly on the care that is taken in obtaining the specimens of blood and the skill of the clinician in interpreting positive results.

The presence of bacteremia and fungemia is an indication of the failure of the host's immune system to localize infection at its primary focus or of the physician's failure to remove, drain, or sterilize that focus. This article reviews the ability of the microbiology laboratory to detect bloodstream infections quickly and efficiently.

Various factors are important in the laboratory detection of bacteremia and fungemia. These include clinical factors, the type of blood culture system, and laboratory processing of blood culture bottles. Most commercial blood culture products have been designed to take these factors into account and, thus, are relatively similar. There are differences, however, and it should not be assumed that these products have identical performance characteristics. To optimize microbial recovery, clinicians should order and collect the proper number of blood cultures at the earliest possible time and before the administration of antimicrobial agents. Clinical microbiologists should select a blood culture system that optimizes the recovery of common microbial pathogens and should use blood culture systems according to the manufacturers' recommendations.

For the first time, clinical microbiologists have the option of purchasing or leasing a variety of automated blood culture systems. Although the performance characteristics of the BACTEC radiometric and nonradiometric blood culture systems are well-documented, we are only beginning to learn the performance characteristics of CMBCS. Well-designed and executed controlled clinical trials are needed to establish performance characteristics and to address issues that are unique to CMBCS. In particular, controlled clinical comparisons of different CMBCS are needed. In terms of the impact on clinical microbiology laboratories, there are no published data to support claims that the use of these systems decreases laboratory workload, staffing, or costs. Although such an impact seems reasonable and even likely, unequivocal documentation of such savings would make it easier for many laboratorians to justify the cost of purchasing or leasing one of these systems. Similarly, there are no published data regarding the long-term reliability of these systems while in clinical use. A documented track record of mechanical and electrical reliability would be useful to clinical microbiologists contemplating the acquisition of a new blood culture system.

Bacteremia and fungemia are encountered commonly and are associated with significant morbidity and mortality. Laboratory detection is therefore of paramount importance. Principles of successful blood culturing, clinically important technical issues, manual and automated detection systems, and interpretation of culture results are reviewed in this article.