Rapid and reliable identification of bacteria directly from blood cultures is important in clinical practice to guide appropriate antibiotic therapy. In this study, the performance of the AccuProbe (Gen-Probe, Inc., San Diego, Calif.) in direct identification of Staphylococcus aureus, Streptococcus pneumoniae, enterococci, and group A and B streptococci from positive blood culture bottles was evaluated by using 6-year routine clinical laboratory blood culture material from Päijät-Häme Central Hospital, Lahti, Finland. With the enterococcal and group A and B streptococcal probes, the diagnostic performance of the test was excellent at a cutoff value of 50,000 relative light units (RLU) as recommended by the manufacturer. However, with the S. aureus probe, although the specificity was very high (99.8%), the sensitivity was low (72.4%). To improve the clinical usability of the direct AccuProbe identification, optimal cutoff values for the individual AccuProbe tests were defined by using receiver-operating characteristic analysis. Consequently, cutoff values for S. aureus and S. pneumoniae tests were adjusted to 30,000 RLU and for enterococci and to 55,000 RLU for group A and B streptococci. With these adjustments, the performance of the AccuProbe tests, especially that for S. aureus, was significantly improved.Rapid and reliable identification of bacteria from blood culture is indisputably one of the most important functions of a clinical microbiology laboratory. Bloodstream infection is a potentially life-threatening condition that requires rapid, appropriate therapy. The increasing rate of nosocomial infections during the past decade (21), together with the threat from multiresistant infectious agents, further highlight the importance of early identification and tailored antimicrobial therapy.