Evaluation of a rapid test panel, the API Strep 20, the BD Phoenix and VITEK 2 automated instruments, and Raman spectroscopy for species identification of Enterococci

Abstract number: 1732_68

Top J., Scholtes M., Maquelin K., Willems R., Bonten M., Wolfhagen M., Hegge R., Asbroek M., Bruins M., Leverstein-van Hall M.

Objectives: The identification of vancomycin-resistant enterococci (VRE) has become an important component of infection-control programmes. The differentiation between vanA/B-VRE [high-level and transferable vancomycin resistance in Enterococcus faecium and Enterococcus faecalis (Efe/Efa)] and vanC-VRE [intrinsic low-level vancomycin-resistance in Enterococcus casseliflavus and Enterococcus gallinarum (Ecas/Egal] is relevant since in contrast to vanA/B-VRE, vanC-VRE have not been implicated in outbreaks. Furthermore, vancomycin treatment failure has been associated with infections caused by vanC-VRE. Differentiation of vanC-VRE from other enterococcal species by adequate identification is therefore relevant since low-level resistance may not be detected using the CSLI breakpoints.

In the current study we evaluated a simple rapid test panel (RTP), the BD Phoenix and VITEK 2, the API Strep 20 and Raman spectroscopy for their accuracy to identify clinical relevant enterococcal species and their sensitivities and specificities to distinguish Efe/Efa from vanC positive enterococci.

Method: In total 96 clinical enterococcal strains comprising 8 different species were analysed. A genotypic test based on the sequence of the rpoA gene was used as reference method. The phenotypic test panel provided a species identification within 4 hours, testing the reduction of litmus milk, acidification of arabinose and methyl-alpha-D-glucopyranoside, hydrolysis of L-arginine, pigment production and motility. Raman spectroscopy is a relatively new identification method under development yielding results within 1 minute.

Results: The table shows the accuracy of the different tests relative to the sequence-based identification varied between 86% (BD Phoenix) and 96% (Raman). The best method to distinguish Efe/Efa from vanC positive species was the RTP with a sensitivity and specificity of 100% The API method performed poorly with a relative high sensitivity of 98.6% but very low specificity (41.7%). With this method 7/12 Ecas/Egal were identified as E. faecium.

Identification methodTotal no. (%) accurateDiscrimination between Efe/Efa and Ecas/Egal
Sens. (%)Spec. (%)  
Rapid phenotypic tests88 (92)100100
API Strep 2035 (39)93.641.7
BD Phoenix83 (86)98.4100
VITEK 284 (88)98.591.7
Raman spectroscopya244 (96)99.536.1
aA selection of 85 strains were tested in triplo.

Conclusions: All methods were comparable regarding the identification of enterococci. However, from the routine laboratory tests the RTP was the most rapid and reliable method to distinguish Efe/Efa from Ecas/Egal. Raman spectroscopy is a very promising fast alternative. The API revealed a high percentage of false positive E. faecium identifications, which may result in unnecessary infection control interventions.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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