C2-276. Evaluation of Vancomycin (VAN) Potency Trends (“Creep”) Against Methicillin-Resistant S. aureus Collected in Nine United States Hospitals Over Five Years (2002- 2006)
Session: Poster Session: Vancomycin MIC-CREEP and Resistance in Staphylococcus aureus
Saturday, October 25, 2008: 12:00 AM
Room: Hall C
Background: VAN MIC creep has been reported by some institutions but not proven in large surveillance studies. We evaluated the possible MIC creep occurrence when testing VAN and daptomycin (DAP) against MRSA by precise CLSI methods. Methods: 9 hospitals (1 / CDC region) randomly selected bloodstream MRSA strains (target, 40/year) from 2002-2006. MICs were determined by CLSI broth microdilution (BMD) using incremental dilutions (up to 7 between each log2 dilutions). Isolates with VAN MIC >1 μg/ml were typed by PFGE. Results: VAN MIC mode was 0.625 μg/ml in all centers and 73% (69-77%) of strains had VAN MIC between 0.563 and 0.688 μg/ml. No yearly variation on the central tendency of VAN MIC of the wild-type population was observed in any medical center; however, when analyzed by geometric mean, VAN MIC showed increases in 3 and decline in 3 centers. DAP MIC mode varied from 0.156 (2003-2005) to 0.219 μg/ml (2002 and 2006) and 83% (80-89%) had MIC between these values. Among PFGE typed strains, 38 of 55 (69%; 7 hospitals) showed a pattern similar to USA100; which represented all strains from 2 hospitals and 64-80% of strains from 4 other hospitals; only 1 strain (2%) was USA300.
Year (no.)% of strains at vancomycin MIC (μg/ml) value of:
≤0.3750.406-0.50.5630.6250.6880.75-1>1
2002 (342)2.66.78.536.824.018.13.2
2003 (365)1.16.69.645.222.513.11.6
2004 (347)2.96.18.939.819.918.73.7
2005 (380)2.47.910.841.622.111.63.7
2006 (366)1.14.19.344.021.015.83.0
Conclusions: Perception of VAN MIC creep may vary according to the methods used to analyze the data. Geometric mean MIC data revealed 3 of 9 sites had an MIC Creep over the 5 year period which was not evident using modal MIC values. Prevalence of strains with VAN >1 μg/ml was very low, with no increase trend but related to clonal occurrence (USA100).
Ajit Limaye, MD, FIDSA, University of Washington, Seattle, WA, David Snydman, MD, FIDSA, Tufts New England Medical Center, Boston, MA, Douglas Fish, MD, Albany Med. Coll., Albany, NY, George Pankey, MD, Ochsner Clinic Foundation, New Orleans, LA, Helio Sader, MD, PhD, JMI Lab., North Liberty, IA, James Rahal, MD, New York Hospital Medical Center of Queens, Flushing, NY, Ken Waites, MD, Birmingham Veterans Affairs Medical Center, Birmingham, AL, Lisa Steed, PhD, Medical University of South Carolina, Charleston, SC, Michael Rybak, PharmD, MPH, Paul Fey, PhD, University of Nebraska Medical Center, Omaha, NE, Ronald Jones, MD, Microbiology, JMI Laboratories, North Liberty, IA and  H. S. Sader,
Cubist Pharmaceuticals, Inc. Role(s): Grant Investigator, Received: Research Grant.