I am writing this for a group of outpatient laboratories. They have recently started using the Vitek 2 for susceptibility testing. Occasionally, they have the following result for E. coli in urine infections: ampicillin - resistant, cephalothin - susceptible (MIC = 8 ug/ml), and cefuroxime axetil - intermediate (MIC 16 ug/ml). The Vitek Expert System reports the mechanism of resistance as "acquired beta-lactamase". How should they be reporting the results? Should they be changing the result of cephalothin to intermediate, or leaving it as is?
 

The pattern you describe is unusual in that cefuroxime should not be intermediate if cephalothin is susceptible. Yet, both are one dilution from the breakpoint which suggests that repeating the test may resolve the issue. While this may be an acquired beta-lactamase (it is difficult to know without seeing the entire antibiogram), there are no standard NCCLS recommendations for changing cephalothin results based on ampicillin and cefuroxime results and we would not recommend doing so.