Time to Blood Culture Positivity Supports Shorter Rule Outs in a Level IV NICU

Antibiotic therapy is typically initiated and continued for 36 or 48 hours in neonatal patients while awaiting blood culture results. Our study aims to determine time to positivity (TTP) of positive blood cultures in a level IV neonatal intensive care unit (NICU) to determine the appropriate duration of empiric antibiotics.

This is a retrospective study of all patients admitted to the NICU with a positive blood culture from January 2021 to March 2025. Only the first positive blood culture per patient was included in the analysis unless a new organism was identified in subsequent cultures or if 14 days had passed since the previous positive blood culture. Polymicrobial cultures were excluded from analysis unless a subsequent culture demonstrated continued positivity with a single organism.

A total of 63 positive blood cultures from 59 patients were included in the analysis. The median TTP for gram-negative and gram-positive definite pathogens was 9 hours (IQR 7.79, 10.52) and 16.2 hours (IQR 13.76, 18.57), respectively. At 24 hours, 97% (30/31) of definite pathogens were reported as positive, and at 36 hours, 100% (52/52) of definite and possible pathogens were reported as positive.

All blood cultures with definite and possible pathogens were reported as positive by 36 hours. A 36-hour course of empiric antibiotics should be considered in neonates without signs of site-specific infection and without other pending cultures. Shortening courses to 24 hours may also be reasonable, particularly for gram-negative pathogens.