Comparison of the automated Bactec NR-660 with a conventional blood culture system

Blood Culture Contaminants in a Paediatric Population Retrospective study from a tertiary hospital in Oman

OBJECTIVES

Most children presenting with febrile illness require a blood culture to determine the causative organism as well as its sensitivity to antibiotics. However, false-positive results lead to unnecessary hospitalisations, prescriptions and tests. This study aimed to evaluate the impact of false-positive blood cultures among a paediatric population at a tertiary hospital in Oman.

METHODS

This retrospective study included all 225 children <13 years old with positive blood cultures who presented to the Sultan Qaboos University Hospital, Muscat, Oman, between July 2011 and December 2013. Blood cultures were reviewed to determine whether they were true-positive or contaminated.

RESULTS

A total of 344 positive blood cultures were recorded during the study period, of which 185 (53.8%) were true-positive and 159 (46.2%) were contaminated. Most true-positive isolates (26.5%) were coagulase-negative Staphylococcus spp. (CONS) followed by Escherichia coli (9.7%), while the majority of contaminated isolates were CONS (67.9%) followed by Streptococcus spp. (6.9%). Children with contaminated cultures were significantly younger (P <0.001) while those with true-positive cultures required significantly more frequent hospital admissions, longer hospital stays and more frequent antibiotic prescriptions (P <0.001 each). Chronic illness and mortality was significantly more frequent among those with true-positive cultures (P <0.001 and 0.04, respectively). While white blood cell and absolute neutrophil counts were significantly higher in true-positive cultures (P <0.001 each), there was no significant difference in C-reactive protein (CRP) level (P = 0.791).

CONCLUSION

In this population, CRP level was not an adequate marker to differentiate between true- and false-positive cultures. A dedicated well-trained phlebotomy team for paediatric patients is essential.

Determination of growth value thresholds for BACTEC PLUS aerobic blood culture vials

Growth value thresholds used to identify positive blood culture vials can be defined by users for each BACTEC NR-660 bacteremia detection instrument. Growth values were compared with the recovery of organisms from vials flagged as positive during the testing of 3.056 high-volume vials containing aerobic (BACTEC PLUS 26) medium over a 2-month period. Results showed that optimal threshold values for our use of these vials varied from those recommended by the manufacturer; if the thresholds defined from these data had been used during the study period, total vials flagged as positive from which no organisms were recovered (false alarms) would have been reduced from 181 (5.9/100 vials tested) to 71 (2.3/100 vials tested), with a minimal decrease in the identification of vials containing usual or occasional pathogens (hits). Adjustments of growth value thresholds by the individual user can make the use of BACTEC instruments more efficient by decreasing further processing of vials from which no organisms are recovered.