Volume of blood required for culture positivity in an experimental bacteremia model

Objective. To determine the minimum volume of blood and absolute number of organisms required for detection of bacteremia and fungemia by an automated colorimetric blood culture system (BacT/Alert, Organon Teknika Corp., Durham, N.C.). Design. Common neonatal pathogens, E. coli, S. agalactiae (GBS) one ATCC strain and one "clinical isolate," S. epidermidis, and C. albicans were seeded into blood to make low colony count (1-3 cfu/ml) and ultra-low (<l cfu/ml) bacteremia or fungemia. Microorganisms were grown, suspended and diluted to 10^2-3CFU/ml. 1.0ml, 0.1ml volumes of each bacteria suspension were mixed in aliquots of 100 ml of fresh, single donor, whole blood (low and ultra-low bacteremia/fungemia). Quantitative colony counts on plated media verified the colony counts of the bacterial suspensions. For each organism 96 culture bottles were inoculated with either 0.25, 0.5, 1.0 or 4.0 ml of the two seeded blood concentrations. Blood culture bottles were incubated in the BacT/Alert device for 5 days and time to positivity noted when applicable. All bottles were subcultured on plated media. Data analysis. The Poisson statistic was used to calculate the probability of finding at least one viable CPU per culture bottle inoculated. The number of positives per group was divided by the probability of ≥1 organism present to give the positivity index. Results: Plated subculture identified no growth of organims not detected by the calorimetric detection system. The false positive rate for the automated device was <1%. The positivity index and median time to detection in hours for each bacteria is listed in the table. There was a statistically significant correlation with time to positivity and inocula volume (p < 0.01), but the differences were not clinically important. Conclusions: If there are one to two viable CFUs contained in the blood inoculated into culture media, the BacT/Alert system will detect growth rapidly. Unless an infant is at risk for sepsis with a colony count less than 4 CFU/ml, then a minimum 0.5cc inocula of blood into the culture media should be adequate for sensitive and timely detection of bacteremia. Organism P.L. Low UL GBS-clinical 1.13 12.3 12.5 GBS-ATCC 0.96 16.0 16.7 S. epidermitis 0.94 13.0 13.5 C. albicans 0.97 28.2 30.2 E. coli 0.95 13.0 13.5 P.I.= positivity index, UL=ultra low.