A comparison of a conventional and a radio-metric examination of clinical blood cultures with respect to recovery rate and detection time of microorganisms.

The results from a comparative investigation of two different blood culture systems with respect to the recovery rate and the detection time are presented. Patients from whom enough blood volume could be drawn to fill two or more Venoject tubes, were consecutively included in the study. The blood was equally divided between our conventional system (4 tubes of nutrient broth, 4 tubes of semisolid nutrient agar and 4 tubes of semisolid thioglycollate agar) and a radiometric system, Bactec (one aerobic and one anaerobic vial). From a total of 1252 blood cultures, one or both systems detected 170 positive cultures (13.6%) from 140 patients. Of a total of 127 positive blood cultures representing bacteremia, the dominating species were Escherichia coli and Staphylococcus aureus, accounting for 31 and 27% respectively. The radiometric system detected 95% and the conventional system 82% of all clinically relevant isolates (p less than 0.001). Especially E. coli and Klebsiella pneumoniae were detected more frequently in the radiometric system. The radiometric system had a shorter average detection time than the conventional system (1.0 versus 1.7 days). This may have therapeutical and prognostical consequences for patients with bacteremia. One fourth of all positive blood cultures was due to contamination. The majority of the contaminants, mainly coagulase negative staphylococci, were found in the radiometric system, as were false positive cultures. This comparative study demonstrated positive and negative sides of both systems tested.