Automated continuously monitoring blood culture systems brought many advantages into laboratory practice such as rapid detection of agents and labor reduction. Delay in the specimen transport is one of the problems that may effect the culture positivity in clinical laboratories. To evaluate the difference between effect of delayed entry into blood culture systems [Bactec 9240 (BD) and BacT/Alert 3D (BA)] on detection of bacterial growth in blood culture media (Bactec 92 F, 93 F and BacT/Alert FA, BacT/Alert FAN), standard inoculums (25 cfu) of micro-organisms frequently isolated from blood cultures; Staphylococcus aureus, Escherichia coli, Enterococcus faecalis, Acinetobacter baumannii, Haemophilus influenzae, Bacteroides fragilis and Streptococcus pneumoniae were inoculated into blood culture bottles which contained 2 mL of sterile human blood. The bottles were cultured using two instruments after they were stored at room temperature and at 35^[compfn]C incubators for 0, 4, 6, 12, 24 and 48 h. Five sets (60 pairs) of each bacteria were studied for each temperature. All the positive and negative cultures (after 5 days ) were recorded and subcultures were performed. None of the cultures were false negative in the first 12 h of delay except two strains (S. pneumoniae and H. influenzae) in BA, after preincubation at 35^[compfn]C. At 24 h of delay, false negativity was 14/60 for BD (E. coli 4, A. baumannii 3, E. faecalis 4 S. pneumoniae 3), 8/60 for BA (S. pneumoniae 8). False negative results were from bottles preincubated at 35^[compfn]C in BD.

At 48 h of delay, false negativity rate was 34/70 for BD (E. coli 10, B. fragilis 10, E. faecalis 6, S. pneumoniae 5, A. baumannii 2, H. influenzae 1), 15/70 for BA (S. pneumoniae 9, A. baumannii 3, B. fragilis 3). For delayed preincubation (24–48 h) false negativity was more common in BD especially for storage at 35^[compfn]C. BA had difficulty to detect S. pneumoniae starting from 24 h preincubation. Failures to detect microbial growth is noticed after 24 h for both systems. This may not be a disadvantage in clinical laboratory practice at institutions where specimen transportation work regularly.