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Evaluation of the Etest GRD strip for identification of hGISA

Abstract number: P1737

Leonard S., Rossi K., Rybak M.

Objective: Continued selective pressure from glycopeptide use has led to non-susceptible strains of Staphylococcus aureus including hGISA. Infections with hGISA are associated with prolonged bacteraemia and vancomycin (VA) failure. The gold standard for identification of hGISA is population analysis profile – area under the curve assay (PAP-AUC) where the isolate is plated on agar plates with increasing VA concentrations and resulting CFU/mL are plotted versus VA concentration (Wooton et al. J Clin Microbiol.2007;45:329–32.). This method is time consuming and labor intensive and not suitable for use in clinical laboratories. This study compared a new Etest gradient strip (GRD) for detection of hGISA to PAP-AUC and the Etest macromethod (MET) (Walsh et al. J Clin Microbiol. 2001;39:2439–44.).

Methods: 93 clinical hGISA and 25 clinical non-hGISA strains defined by PAP-AUC were tested with GRD strip consisting of a double-sided gradient with VA and teicoplanin (TP), according to the manufacturer's instructions. An inoculum suspension (0.5 McFarland turbidity) was streaked on a Mueller-Hinton (MH) and MH + 5% sheep blood (MHB) agar plates. Etest VA strip was applied to the MH plate and GRD strip to the MHB plate. VA MIC results were read at 24 h and GRD results were read at 24 and 48 h. For GRD, VA or TP results geqslant R: gt-or-equal, slantedmg/mL and Etest VA MIC of leqslant R: less-than-or-eq, slant4 was interpreted as positive for hGISA. MET was done as previously described.

Results: Sensitivity and specifity for GRD was 81% and 96% at 24 h and 93% and 79% at 48 h, respectively and for MET, 76% and 100% at 48 h. For hGISA strains, VA MIC50/90 were 1.5/2 mg/mL and for non-hGISA, corresponding values were 1/1.5.

Conclusion: Etest GRD correctly identified 81% of hGISA at 24 h with high specificity and sensitivity improved to 93% at 48 h. Further evaluation of this test is warranted.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Subject:
Location: Barcelona, Spain
Presentation type:
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