Comparative evaluation of the oxoid signal and Roche Septi-Chek blood culture systems

The Oxoid Signal blood culture system (Oxoid USA, Inc., Columbia, Md.) was compared with the Roche Septi-Chek system (Roche Diagnostics, Div. Hoffmann-La Roche Inc., Nutley, N.J.), with the latter consisting of a tryptic soy broth (R-TSB) bottle with an attached agar slide unit and a Columbia broth bottle. A total of 5,034 cultures with equal volumes of blood in each bottle were processed. Overall, more organisms were recovered in the R-TSB bottle than in the Signal bottle, with significantly more aerobic organisms (Pseudomonas spp., Acinetobacter spp., and yeasts) recovered in the R-TSB bottles and anaerobes and viridans group streptococci recovered in Signal bottles. Approximately equivalent numbers of organisms were recovered in the Signal and Columbia broth bottles. The times of detection were essentially identical with the three blood culture broth systems. During the study, 30.6% of the Signal bottles had a positive indicator of growth, of which 1,103 (71.7%) were false-positive cultures. Additionally, nonviable organisms resembling streptococci were observed in 13.7% of the Signal bottles that were Gram stained and in unioculated blood culture bottles. With appropriate modifications of the preparation of the media, the latter problem can be eliminated.