Epidemiology of antimicrobial resistance among Gram-positivesMonday, April 21, 2008, 13:30 - 14:30
Evaluation of the Etest GRD strip for identification of hGISAS. Leonard, K. Rossi, M. Rybak (Detroit, US)
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 bacteremia 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 >= 8 µg/mL and Etest VA MIC of <= 4 was interpreted as positive for hGISA. MET was done as previously described.
Results: Sensitivity and specifity for GRD was 81% and 96% at 24h and 93% and 79% at 48h, respectively and for MET, 76% and 100% at 48 h. For hGISA strains, VA MIC50/90 were 1.5/2 µg/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.