Comparison of BacT/Alert with Signal blood culture system

The BacT/Alert (Organon Teknika Corp., Durham, N.C.) is an automated blood culture system. It is based on the detection of CO2 by means of a colorimetric sensor internally attached to the bottom of culture bottles. The aerobic and anaerobic media of this system were compared with one bottle of the Signal system (Oxoid Ltd., Hampshire, United Kingdom). At bedside, 20 ml of blood was drawn from each adult patient. The two BacT/Alert bottles were inoculated with 5 ml of blood each; the Signal bottle was inoculated with 10 ml. A total of 5,284 sets (2,483 patients; 2.1 cultures per patient) consisting of three bottles each were evaluated, of which 781 sets (14.8%) revealed microorganisms (n = 892); 642 of these were considered to be pathogenic. Significantly more (P < 0.0001) pathogens were isolated from the two BacT/Alert bottles together (n = 584) than from the single Signal bottle (n = 515). Escherichia coli (P = 0.007), gram-negative bacteria other than members of the family Enterobacteriaceae or Pseudomonas spp. (P = 0.006), and yeasts (P = 0.02) were isolated more often from both or either BacT/Alert bottle. Comparing the systems in terms of 388 different organisms per septic episode, the difference between BacT/Alert and Signal was significant for the total number of septicemia cases (P = 0.003). More contaminants grew in the BacT/Alert system (173 versus 116; P = 0.0001). False-positive indications were more frequent in the BacT/Alert system, 198 (3.7%) aerobic bottles and 57 (1.1%) anaerobic bottles, than in the Signal bottles, 24 (0.5%) bottles. Pathogens could be detected significantly earlier (P < 0.0001) in the BacT/Alert system than in the Signal system. The BacT/Alert instrument with two bottles allowed earlier detection as well as the isolation of more microorganisms than the manual, one-bottle Signal system.