Increasing Vancomycin resistance in MRSA and its impact on treatment.

EMBARGOED UNTIL:  Monday, September 8, 2014, 11:00 A.M. EDT

(Session 157, Paper C-1382)

Erina Khadka Kunwar
NJ Med. Sch., Rutgers, Newark, NJ, United States

Phone: 973 668 3466

While increment in MICs were observed in both MRSA and MSSA, there was no detectable increase in duration of bacteremia with time. This would hence argue for continued use of Vancomycin as the initial therapy for MRSA until MIC for an isolate is known.

Increasing prevalence of Methicillin resistant Staphylococcus aureus has been found in the recent years. Currently, Vancomycin is the antibiotics most frequently used to treat MRSA infection. However, treatment failure with  Vancomycin have been reported and has been attributed to increase in MICs of Vancomycin over time which is described as “Vancomycin Creep”. However the literature on clinical outcomes on the patients with infection caused by MRSA is inconsistent. Our study was performed to see if failure of treatment by Vancomycin has been observed and associated with the increase in Vancomycin MICs.

This study was performed at the VA New Jersey Health Care System, East Orange, NJ and funded by the Division of Infectious Diseases. In this retrospective chart analysis, 2267 blood cultures  done in 911 patients were positive for Staphylococcus aureus. Isolates of Staphylococci were identified using  Vitek. MICs of vancomycin were determined by the E-test using CLSI standards.  Susceptibility breakpoint for Vancomycin was 2 mg/dL.  MICs were available dating back to 2010 and hence changes in MICs were analyzed during this time period. Of the cultures analyzed 1086 were MRSA and 983 were MSSA. Treatment failure has been described as persistent bacteremia on appropriate antibiotic therapy. A correlation and regression analysis of duration of bacteremia versus time was done and no increase in duration versus time was observed. When MICs of MSSA vs MRSA were compared, MSSA median MIC shifted from 1.5 towards 2 and developed higher degree of Vancomycin resistance and MRSA median MIC shifted from 1 towards 1.5 (note less resistant than MSSA). Although the approach of using Vancomycin for the treatment of MRSA is controversial, based on our study, Vancomycin is still indicated as a first line therapy for treatment of MRSA until the MIC for the particular isolate is known.