246. Impact of Cephalosporin and Carbapenem Breakpoint Revisions on Enterobacteriaceae Susceptibility Rates
Session: Poster Abstract Session: Antimicrobial Susceptibility and Resistance
Friday, October 21, 2011
Room: Poster Hall B1

Background: 

The Clinical and Laboratory Standards Institute (CLSI) lowered Enterobacteriaceae breakpoints for several cephalosporins and the carbapenems in 2010.  We evaluated the impact of these breakpoint changes on in vitro Enterobacteriaceae susceptibility rates.

Methods:

New Enterobacteriaceae susceptibility breakpoints for cefazolin (≤2 mg/mL), ceftriaxone (≤1mg /mL), meropenem (≤1mg/mL), and ertapenem (≤0.25 mg /mL) have been used at Mayo Clinic since September, 2010. Susceptibility rates were compared using chi-square or fisher's exact test for the periods before (1/1/2009 to 12/31/2009) and after (9/1/2010 to 3/31/2011) the breakpoint changes were implemented. MIC testing was performed using agar dilution.

Results:                                             

Cefazolin susceptibility rates decreased significantly for all tested Enterobacteriaceae (96% to 80% for C. koseri, 90% to 65% for E. coli, 79% to 18% for K. oxytoca, 93% to 85% for K. pneumoniae, and 92% to 2% for P. mirabilis).  Smaller decreases in susceptibility were noted for ceftriaxone against Enterobacter species and Morganella morganii and for ertapenem against Enterobacter species.  No significant decreases in susceptibility were noted for meropenem.

Percentage Susceptible at New and Previous Breakpoints

 

Cefazolin

Ceftriaxone

Ertapenem

Meropenem

Microorganism

2010/11

< 2

2009

< 8

2010/11

< 1

2009

< 8

2010/11

< 0.25

2009

< 2

2010/11

< 1

2009

< 4

Citrobacter fruendii complex

 -

 -

84

86

97

99

99*

93

Citrobacter koseri

80*

96

98

100

100

100

100

100

Enterobacter aerogenes

 -

79

87

94*

99

98

100

Enterobacter cloacae complex

 -

 -

81*

87

85*

99

100

100

Escherichia coli

65*

90

93*

96

99*

100

100

100

Klebsiella oxytoca

18*

79

97

99

100

100

100

100

Klebsiella pneumoniae complex

85*

93

97

96

99*

100

100

100

Morganella morganii

 -

 -

85*

99

99

99

100

100

Proteus mirabilis

2*

92

99

100

100

100

100

100

* = statistically significant change      

Conclusion:

The CLSI breakpoint revisions resulted in significant decreases in susceptibility, especially for cefazolin. Clinical outcome data are needed to support or refute cefazolin breakpoint changes.

 


Subject Category: A. Antimicrobial agents and Resistance

Lynn Estes, PharmD1, Robin Patel1, Peggy Kohner, BS1 and Ritu Banerjee, MD, PhD2, (1)Mayo Clinic, Rochester, MN, (2)Division of Pediatric Infectious Diseases, Mayo Clinic, Rochester, MN

Disclosures:

L. Estes, None

R. Patel, None

P. Kohner, None

R. Banerjee, None

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.