Yesterday, I wrote a post about a new version of CLSI’s Performance Standards for Antimicrobial Disk and Dilution Susceptibility Tests for Bacteria Isolated From Animals. There are some major changes in this update, and I sympathize with the diagnostic labs that now have to update their testing and reporting. It won’t happen overnight, because some may need to develop different testing panels to account for some of the different breakpoints, and they’ll need to change their reporting systems.

While we want to push labs to update their methods and reporting ASAP, we still have to manage patients in the interim. My next few posts will explain how I’ll be looking at interpretation of antimicrobial susceptibility testing results in the meantime.

I’ll start with chloramphenicol and staphylococci in dogs. Our typical methicillin-resistant Staphylococcus pseudintermedius (MRSP) tends to be resistant to multiple antimicrobial classes beyond beta lactams, and therefor has limited treatment options, so chloramphenicol susceptibility often gets included in reports for this bug.

The old breakpoints for chloramphenicol and staphylococci in dogs were:

  • Susceptible < 8 ug/mL
  • Intermediate 16 ug/mL
  • Resistant > 32 ug/mL

The *new* breakpoints for chloramphenicol and staphylococci in dogs are:

  • Susceptible < 2 ug/mL
  • Intermediate 4 ug/mL
  • Resistant > 8 ug/mL

I don’t think the intermediate category means much here, since chloramphenicol isn’t a drug for which we can safely increase the dose. So, practically speaking, the bacterium must have an MIC < 2 ug/mL for chloramphenicol to have a chance at being effective.

One big problem the labs will have is that at least one of the plates commonly used for susceptible testing of bacterial isolates from dogs won’t be able to differentiate susceptible from resistant to chloramphenicol based on these new guidelines, because the lowest concentration it contains is 8 ug/mL of chloramphenicol.

  • If there’s no growth in the 8 ug/mL well, then the bug was inhibited at that concentration and we know the MIC is 8 ug/mL or lower; but we won’t know if the true MIC is 8 ug/mL (resistant), 4 ug/mL (intermediate) or < 2 ug/mL (susceptible).
  • If the bacterium grows in the 8 ug/mL well, then the MIC is clearly greater than 8 ug/mL, meaning the bug will be clinically resistant to chloramphenicol.

Labs will need new plates with (at a minimum) 1, 2, 4 and 8 ug/mL chloramphenicol wells.

So, what do we do in the interim? We have to look at lab reports to see if an MIC is reported.

If your lab reports MICs:

  • Look at the MIC, not just S/I/R category.
  • If the MIC is 4-8 ug/mL, the bug is resistant, even if the report still lists it as “susceptible.”
  • If the MIC is “<8 ug/mL” or “<4 ug/mL,” we have no way to tell if the bug is resistant or susceptible. Since a large percentage of S. pseudintermedius from dogs will have MICs that fall in the 4-8 ug/mL range, odds are pretty good that treatment will fail if all we know if that the MIC is <8 ug/mL, so I’d consider it resistant to be on the safe side (see examples below).

If your lab does not report MICs:

  • I’d consider the bug resistant to chloramphenicol, since most staphylococci are resistant to this drug.
  • Ask your lab to start reporting MICs. As we learn more and get better guidelines, MICs are becoming more useful, and therefore important to include on antimicrobial susceptibility reports.