Discontinuation of antibiotics in neonates: When does the blood (BL) culture (CX) become positive?

Antibiotics are frequently administered to infants in the neonatal intensive care unit (NICU) and newborn nursery (NBN) while awaiting BL CX results. In an effort to decrease exposure of infants to antibiotics and minimize the risk of emergence of antibiotic-resistant pathogens, we analyzed the timing to positivity of BL CX obtained from infants in NBN/NICU of a large, county hospital. From 1/95-12/31/97, there were 576 BL CX that yielded ≥1 bacterial isolate from 158 infants. Their clinical course and laboratory values were reviewed and the decision was made as to whether the isolate(s) was responsible for the sepsis episode or represented a contaminant. The time that the microbiology laboratory received the BL CX and the time that the BL CX yielded bacterial growth was analyzed. All BL CX were processed using the BacT/Alert® Microbial Detection System. Of the 576 ⊕ BL CX, 341(59%) were assessed to represent true pathogens; these occurred in 158 infants. Of the 341 ⊕ BL CX, 30 (9%) were from NBN infants and 311(91%) from NICU infants. There were 38 episodes (23 group B streptococci [GBS], 9 Gram-negative bacilli, 6 other) in 38 infants with early-onset (EO) infection (0-72 hrs after birth) and 135 episodes (62 coagulase-negative staphylococci [CONS], 33 Gram-negative bacilli, 7 Staphylococcus aureus, 5 GBS, 33 other) of late-onset (LO) infections (>72 hrs) in 120 infants. Since many infants had 2 BL CX performed per sepsis episode, there were more bacterial isolates than episodes of bacteremia Overall, the time to positivity of the BL CX in the EO group was 15±6 hrs (range 4-33 hrs; 99% by 25 hrs) while it was 17±9 hrs (range 6-46 hrs) in the LO group. In the LO group, 229(90%) were ⊕ by ≤24 hrs, 20(8%) by >24 to 36 hrs while an additional 4(2%) by >36 hrs (maximum 46 hrs). Specifically, there were 36 GBS isolates which required 14±6 hrs of incubation, 39 E. coli isolates which required 10±2 hrs, and 116 CONS isolates which required 20±7 hrs. In addition, there were 12 fungal isolates which required 20±9 hrs of incubation. These data suggest that the majority of bacterial BL CX in neonates become ⊕ within 24 hrs, and that antibiotic therapy can be discontinued by 33 hrs in infants with possible EO infection and in 45 hrs among those with suspected LO infection.