Comparison of Phenotypic with Genotypic Procedures for Confirmation of Coagulase-Negative Staphylococcus Catheter-Related Bloodstream Infections

Abstract

ABSTRACT We sought here to review the present definition of catheter-related bloodstream infections (CR-BSI) due to coagulase-negative staphylococci (CNS) by comparing the routine phenotypic methods with a genotypic procedure that considers different morphotypes. Our phenotypic characterization of CNS isolates included routine identification with biotype and antibiotype. The genotypic diagnosis was based on longer incubation periods with the consideration of all morphotypes and molecular typing by pulsed-field gel electrophoresis techniques. We prospectively selected 61 episodes of suspected CR-BSI by CNS occurring during 1 year, based on the presence of a compatible clinical setting and the isolation of one or more CNS from blood and catheter tip. Of these episodes, 47 (77%) were identified as true episodes of CR-BSI based on the presence of microorganisms of the same genotype in the blood and on the catheter tip. The sensitivity, specificity, positive predictive, negative predictive, accuracy, positive likelihood ratio, and negative likelihood ratio values obtained by different phenotypic microbiological approaches to establish the diagnosis of CR-BSI were as follows: identity at species level (78.7%, 85.7%, 94.9%, 54.5%, 80.3%, 5.51, and 0.25, respectively); identity of species and biotype (59.6%, 92.9%, 96.6%, 40.6%, 67.2%, 8.34, and 0.44, respectively); identity of species and antibiotype (61.7%, 92.9%, 96.7%, 41.9%, 68.8%, 8.64, and 0.41, respectively); and identity of species, biotype, and antibiotype (48.9%, 92.9%, 95.8%, 35.1%, 59%, 6.85, and 0.55, respectively). Our study demonstrates the inaccuracy of the diagnosis of CNS CR-BSI when the current definition based on conventional routine microbiological practice is followed. A new definition of CNS CR-BSI is necessary, at least as an epidemiological and research tool.