There is a paucity of data on the incidence and costs of contaminants in the critical care setting. Schwenzer and colleagues17 studied blood cultures from a surgical ICU over a 2-year period; of 1,411 blood cultures, there were 122 positive culture results, and 51 of these were considered contaminants. Coagulase-negative staphylococci were considered true pathogens only if isolated from both a central line and peripheral venipuncture, from two peripheral blood culture sets, or from patients who were clinically septic and had a single positive culture result from a central venous catheter. Most other skin organisms were considered contaminants. No cost data were provided in this study, and no good estimates exist in current literature. However, it is likely that the costs and consequences of contaminant blood cultures parallel or exceed estimates available for unselected hospital admissions. Differentiating contaminants from true pathogens is often difficult. In one of the earlier studies on this topic, MacGregor and Beaty19 determined that pneumococci, group A Streptococci, Enterobacteraceae, Bacteriodes, Candida, and Haemophilus spp were true pathogens in their series of patients. Diphtheroids, nonhemolytic streptococci, and Bacillus spp were usually contaminants. Polymicrobial infections occurred in injection drug users or in patients who had leukemia or were receiving chemotherapy. In this study, clinical criteria alone were used to determine whether a positive blood culture result represented true bacteremia or contamination. When two or more blood culture samples were drawn, 69% of patients with a clinical course suggestive of bacteremia had repeatedly positive culture results. This number may be deceptively low, since paired blood culture samples were not drawn, and subsequent culture samples were often drawn while patients were receiving antibiotics. In contrast, only 11% of patients classified as having a contaminated blood culture had positive blood culture results on repeated testing, and often different organisms were isolated. This study highlights the importance of drawing multiple (ie, two or more) blood culture samples as an aid to differentiating between true bacteremia and contaminated cultures.