Detection of positive blood cultures by the Bactec NR660. The clinical importance of five versus seven days of testing.

The Bactec Model NR660 device for detection of bacteremia can sample cultures for either five or seven days before the culture is called normal. The authors studied organisms recovered daily during the seven-day cycle of testing and compared isolates recovered on days 6 and 7 with those recovered earlier. They also reviewed patient clinical charts to determine the clinical impact of the organisms recovered on days 6 and 7. Of 9,062 blood culture vials processed in a nine-week period, 547 had positive results (6% of vials and 10.3% of culture sets). Isolates on day 6 or 7 accounted for 19 (3.5%) of the total; 4 of these were believed to be clinically important by the patients' attending physicians. Detection of these four cultures required processing of 9,062 vials twice each. These data suggest that in our patient population the clinical benefit of testing on days 6 and 7 does not justify its cost. Whether this would be true in other settings would have to be determined by the individual hospital.