Time to positivity of neonatal blood cultures. (Original Articles).(Statistical Data Included)

Abstract

Aim--To determine how long it takes neonatal blood cultures to become positive.

Methods--Data were collected retrospectively on 451 positive blood cultures from babies on a tertiary neonatal unit between January 1997 and December 1998. During the study period, the laboratory used the BacT/Alert microbial detection system.

Results--Complete information was available on 416 blood cultures. Twelve became positive after 72 hours, none of which were considered to be clinically significant. Of the 404 remaining cultures, 86% were positive at 36 hours, 96% at 48 hours, and 98.5% by 60 hours. If definite bacterial pathogens are considered alone, the time to positivity was 90% by 36 hours, 93% by 48 hours, and 98% by 60 hours. If definite and possible bacterial pathogens are considered (coagulase negative staphylococci taken as possible bacterial pathogens), the time to positivity was 89% at 36 hours and 97% at 48 hours. The negative predictive value, for isolation of any organism before 72 hours, of a negative blood culture was 97% at 36 hours and 99% at 48 hours. The negative predictive value for the isolation of definite bacterial pathogens only was 99.7% at 36 hours and 99.8% at 48 hours.

Conclusions--A period of 36 hours is enough to rule out sepsis in the asymptomatic neonate, and a three day incubation period is sufficient to detect all clinically important infections using the BacT/Alert microbial detection system.

(Arch Dis Child Fetal Neonatal Ed 2001;85:F182-F186)

Keywords: blood cultures; positivity; sepsis; infection

Infection is an important cause of mortality and morbidity in the neonatal unit. The early symptoms of sepsis are non-specific, and the outlook is considered to be worst in babies in whom antibiotics are started late. (1) Antibiotics are therefore generally given to all babies with clinical or laboratory indicators of infection and babies at high risk of early onset sepsis. A large number of babies who are evaluated for sepsis do not have proven infection. (2 3) This means that most of the antibiotics given to babies on the neonatal unit are given to babies without infection. Inappropriate use of antibiotics has been implicated in the development of multiresistant bacteria in hospitals. (4)

To reduce the antibiotic pressure on the neonatal unit, it is common practice to stop antibiotic treatment if the blood culture is negative at 48-72 hours and the baby does not have any clinical or laboratory indicators of infection. This practice is based on four studies of the time to positivity for neonatal blood cultures. (2 3 5 6) All of these studies are based on relatively small numbers of positive blood cultures (less than 225 in each case), and three of the studies took place before 1990. (2 3 5) Two of the studies used laboratory techniques that have been superseded and did not consider coagulase negative staphylococci as pathogens, (2 5) although these are now the most common organisms isolated in intensive care nurseries. (7) Recently published studies suggest that antibiotic courses may be safely reduced to 24-36 hours in asymptomatic term babies. (8 9)

The aim of this study was to measure the time taken for bacteria to be detected in blood cultures taken from babies on the neonatal unit using the BacT/Alert microbial detection system (Organon Teknika Corporation, Durham, North Carolina, USA). The purpose was to investigate whether antibiotic treatment could be discontinued before 48 hours in babies with negative blood cultures, so reducing antibiotic pressure on the neonatal unit.

Methods

A two year (January 1997 to December 1998) retrospective study of babies was completed on the neonatal unit at Liverpool Women's Hospital, a tertiary level III neonatal unit. A total of 451 positive blood cultures …