469. Vancomycin Minimum Inhibitory Concentrations (MICs) of Methicillin Resistant Staphylococcus aureus (MRSA) Isolates in a Community Hospital Setting as Measured by E-test Vs Automated Reporting
Session: Poster Session: Hospital-acquired and Transplant Infections
Friday, October 30, 2009: 12:00 AM
Room: Poster Hall A
Background: MRSA is an important hospital pathogen for which we have limited treatment options. Vancomycin may have poor efficacy against isolates with MICs in the upper susceptible range (1-2 mg/L). Also, there are increasing rates of infections with MICs in this range (Vancomycin “creep”). Furthermore, heteroresistance, or subpopulations of intermediately susceptible Staphylococci (hVISA), are emerging and may be another predictor of clinical failure. Having accurate MIC data to guide therapy may be an important factor in choosing efficacious treatments.
Methods: We tested 100 MRSA isolates from different patients for their Vancomycin MICs by two different methods, an automated Vitek2 test, and a manual Standard E-test (SET). In addition, a macrodilution Etest (MET) was used to detect hVISA. SET was performed per CLS standards. MET was done the same way except with a 2.0 McFarland standard for the innoculum. Clinical outcomes were assessed in a subset of 59 patients who had been hospitalized.
Results: There were significant differences between the MICs reported on the same isolates by these two different tests. Whereas the Vitek2 reported MICs of < 1 mg/L on all 100 isolates, the standard E-test reported an MIC of 1.5 mg/L in 48 isolates, 2 mg/L in 50, and 3 mg/L in 2 isolates. Only 1 isolate showed evidence of hVISA on MET.
No significant outcomes differences in the patients were found except the mean length of hospital stay was 10 days vs 13.5 days for patients with isolates having MICs of 1.5 vs 2 and higher respectively (p<0.0001).
Conclusion: Methodologies for identifying vancomycin MICs in MRSA give widely varying results. SET shows consistently higher MICs than Vitek2 and may predict clinical outcome better. hVISA is rare in our population. Larger studies comparing MIC methodologies with clinical outcomes are needed.
Paul Carson, MD1, Avish Nagpal, MD1, Robert Nelson, PharmD2, William Newman, MD1, Loren Podoll, PharmD2, Jody Thompson, MD3 and  A. Nagpal, None..
L. Podoll, None..
J. Thompson, None..
R. Nelson, None..
W. Newman, None..
P. J. Carson, None., (1)University of North Dakota, Fargo, ND, (2)North Dakota State University, Fargo, ND, (3)MeritCare Medical Group, Fargo, ND