DOI: 10.1097/CCM.0b013e3181af1bd1

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PMID: 19609126

Issn Print: 0090-3493

Publication Date: 2009/08/01


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Gram-specific quantitative polymerase chain reaction for diagnosis of neonatal sepsis: Implications for clinical practice*

Husn H. Frayha; Avedis Kalloghlian

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Sepsis in neonates, particularly those with low gestational age and birth weight, is a serious disease that is associated with high morbidity and mortality (1). The outcome of sepsis depends, to a great extent, on early identification of affected infants and rapid initiation of appropriate antimicrobial agents against causative organisms (2). The standard of care for infants suspected to have sepsis is to give combination broad-spectrum antimicrobials to cover potential pathogens at a given age or clinical setting. The type and duration of antimicrobials are usually based on results of cultures and other laboratory markers of sepsis, and the clinical condition of the affected infant. The gold standard for the microbiological diagnosis of a bloodstream infection is a positive blood culture. In small infants, the sensitivity of blood culture is low despite the presence of clinical indicators of sepsis (2–11). This may be due to intermittent seeding of bloodstream with low numbers of bacteria, suppression of bacterial growth by prior antibiotics given to the mother or infant, and insufficient volume of blood samples as is common in neonates (3).
In recent years, there have been several reports (4–13) on the use of polymerase chain reaction (PCR)-based assays for early and accurate identification of bacterial deoxyribonucleic acid (DNA) in the blood of neonates suspected or confirmed to have sepsis. These assays rely on PCR amplification of the 16S rRNA gene, a highly conserved gene present in all bacterial species, but absent in humans. As the gene has a number of divergent regions nested within it, PCR has also been targeted for species-specific detection of bacteria in clinical specimens. When compared with blood cultures, the range of the sensitivity of the PCR assays in various studies (5–7, 9, 10) was 66.7% to 100%, specificity was 87.5% to 97.85%, positive predictive value was 47% to 95.4%, and negative predictive value was 75% to 100%.
The impetus for the development of such assays is to improve the rate of microbiological diagnosis of neonates presenting with signs and symptoms of sepsis and decrease the time for identification of a pathogen. As a result, the expectation is to influence the utilization of antibiotics in such infants.
In this issue of the Critical Care Medicine, Chan and colleagues (14) report the results of an evaluation of a quantitative (q) PCR test for identification of Gram-negative and Gram-positive bacterial infections in 218 episodes of suspected late-onset sepsis in preterm infants. The positivity rate of blood culture was 42/176 (23.86%) and qPCR was 33/176 (18.75%). Compared with blood culture, the sensitivity and specificity of the qPCR for Gram-positive infections were 73.7% and 98.5%, and for Gram-negative infections 86.4% and 99.5%, respectively. The qPCR identified correctly the Gram-specific causative pathogens in negative blood cultures in five infants who had intra-abdominal sepsis. The results of the PCR assay were available more rapidly than blood cultures (5–29 hrs vs. 17.2–127 hrs). These results are in line with previous studies (5–7, 9, 10).
Jordan et al (5) compared the performance of a 16S rRNA gene PCR and blood cultures in 548 neonates with suspected sepsis. Twenty-five infants had positive blood cultures (4.6%) and 27 had positive PCR (4.9%). Compared with blood culture, the sensitivity, specificity, positive predictive value, and negative predictive values of the PCR were 96%, 99.4%, 88.9%, and 99.8%, respectively. The turnaround time for PCR results was around 9 hrs. In another study, Jordan et al (8) evaluated 16S rDNA PCR in 1233 near-term infants with suspected or confirmed sepsis. The blood culture positivity rate was 17/1233 (1.38%) and PCR 37/1233 (3%). Compared with culture, the sensitivity of PCR was 41.2%, specificity 97.

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