Over a year ago, I wrote a few posts about changes to Clinical and Laboratory Standards Institute (CLSI) veterinary antimicrobial susceptibility testing guidelines. When a bacterium is grown in a lab and tested for susceptibility to different drugs, the lab looks to these CLSI guidelines to determine whether they should consider the bug susceptible or resistant to each drug. That’s critical information for choosing an effective drug to treat a patient with an infection.

In January 2024, some major changes were made to these guidelines, particularly with reporting susceptibility to enrofloxacin and marbofloxacin in Staphylococcus, E. coli (and related Enterobacterales) and Pseudomonas, as well as chloramphenicol susceptibility in Staphylococcus and E. coli/Enterobacterales from dogs.

I was hoping veterinary diagnostic labs would be able to implement those changes fairly quickly. I had a lot of empathy for labs given the challenges of sourcing different testing panels and making a big change to their methods and IT workflow. Some labs made the change fairly efficiently. However, the labs that likely account for the majority of veterinary diagnostic testing in North America still haven’t made the switch.

I’m still empathetic to their challenges, but I’m seeing too many treatment failures because veterinarians are using drugs that the lab reported should work, when in reality the updated guidelines tell us the won’t (or we can’t interpret the test results properly because of what they tested or reported). That’s a problem, and I have no doubt that animals have died because of it.

So, in addition to my detailed posts from last year about the CLSI breakpoint changes, here’s a (hopefully) quick and easy reference for interpretation of bacterial culture and susceptibility test results: