Kirby Bauer Testing
In January 2010, the Clinical and Laboratory Standards Institute (CLSI) published new minimum inhibitory concentration (MIC) breakpoints for cefazolin against Enterobacteriaceae (Wayne, 2010). These new breakpoints were largely based on data from bloodstream infections in adults and do not necessarily reflect increased intrinsic resistance of E.coli to cefazolin.
A Kirby Bauer (KB) disk diffusion test can be helpful to identify isolate susceptibility to cefazolin or cephalexin based on these new MIC breakpoints. However, utilizing a higher dose of cefazolin (i.e. 100 – 150 mg/kg/day) or cephalexin (i.e. 75 – 100 mg/kg/day) is likely to overcome intermediate susceptibility for Enterobacteriaceae (e.g E. coli, Klebsiella pneumoniae, Proteus mirabilis) in urinary tract infections, including uncomplicated pyelonephritis.
Therefore, the CMH Urinary Tract Infection Clinical Practice Guideline Committee and subject matter experts recommend higher dose cefazolin or cephalexin without the need for KB disk diffusion in most cases. KB disk diffusion is still recommended for patients who do not respond appropriately to empiric treatment with cefazolin/cephalexin or who are excluded from this guideline (e.g., urologic abnormalities, kidney disease/injury, septic shock, or immunocompromised).
Reference
Wayne, P. (2010). Performance Standards for Antimicrobial Susceptibility Testing (20th Informational Supplement-Update). Clinical and Laboratory Standards Institute (CLSI), pp. M100–S22.
These pathways do not establish a standard of care to be followed in every case. It is recognized that each case is different, and those individuals involved in providing health care are expected to use their judgment in determining what is in the best interests of the patient based on the circumstances existing at the time. It is impossible to anticipate all possible situations that may exist and to prepare a pathway for each. Accordingly, these pathways should guide care with the understanding that departures from them may be required at times.