Time to Positivity of Blood Cultures in a Level IV NICU Varies Based on Organism Category and Population Subgroups: Is a 36-hour rule out safe?

Abstract Background: Antibiotics are frequently prescribed in the NICU, with increasing concern for antimicrobial resistance. Time to positivity (TTP) of positive cultures can support earlier antimicrobial cessation. Methods: Retrospective cohort study all positive blood cultures at a Level IV NICU. TTP calculated from date/time collection to when first growth was reported. Results: Of 309 positive cultures from 268 infants, median TTP [IQR] was 21.1 [14.3, 25.2] hours, with 91.2% positive at 36 hours, and 96.1% positive at 48 hours. 77 gram positive pathogens (24.9%), 60 gram negative pathogens (19.4%), and 169 gram positive commensals identified, with 124 (40.1%) considered a contaminant. Sub-group analysis identified different organism characteristics. Death within 4 weeks of positivity recorded in 9.5%. Conclusion: Majority of gram positive and negative pathogens were identified within 36h of blood culture collection. When antimicrobials are started for concern for bacteremia, 36 hours may be sufficient in a Level IV NICU population.