Diagnostic testing is a cornerstone of veterinary medicine that helps us optimize patient care, but there’s a lot of science behind it that people often forget. We collect a sample, send it off for testing and magically get the results, often without putting a lot of thought into what happens at the lab. Labs (should) follow standard guidelines for performing tests of all kinds that are developed to help ensure they provide relevant and accurate results. As with most things (medicine or otherwise), guidance changes over time as we learn more and as new issues are identified. Keeping up with those changes is critical.

The Clinical and Laboratory Standards Institute (CLSI) is an organization that sets standards for microbiology testing in humans and animals. The standards are updated regularly, with some updates being more impactful than others. Hot off the presses (just today!) is the latest update to the main veterinary testing standards document: VET01SEd7E Performance Standards for Antimicrobial Disk and Dilution Susceptibility Tests for Bacteria Isolated From Animals, 7th Edition. This is a major update with some changes that will have significant impacts on how lab results are reported and interpreted, including changes to some of the interpretive breakpoints for susceptibility of certain bacteria to specific antimicrobials.

A core part of antimicrobial susceptibility testing is determining the minimum inhibitory concentration (MIC) of an antimicrobial for a particular bacterial isolate. The MIC always needs to be interpreted in the context of each patient in the context of how much of the antimicrobial we can expect to get at the site of infection when the animal is treated. A bug could be susceptible to low concentrations of a drug (low MIC), but if drug levels we get in the infected tissue are even lower, it might not be effective. Conversely, a bug might has a fairly high MIC, but if the drug reaches the site of infection at even higher levels, it might still work.

Breakpoints consider how much drug is required to inhibit the bacterium and how high the concentration of drug is expected to reach in the body. The breakpoint is the MIC where things tip between susceptible and resistant (or susceptible, intermediate and resistant).

New “susceptible, dose dependent” (SDD) category

One major change in the updated lab standards is that they now provide guidance for a susceptible, dose dependent (SDD) category for certain bug/drug combinations. This goes beyond the familiar susceptible, intermediate or resistant classification. The SDD category considers the potential to use higher drug doses and how that impacts the bacterium’s susceptibility. That’s relevant for some drugs where we can use higher (but still reasonable and safe) doses.  

Take for example the guidance regarding enrofloxacin and both staphylococci and Enterobacterales (i.e. E. coli and relatives) in dogs.

The previous version of the lab standards said:

  • MIC <0.5 ug/mL: susceptible
  • MIC 1-2 ug/mL: intermediate
  • MIC >/=4 ug/mL: resistant

Two things have now changed. One is that the breakpoints have been lowered a bit. The other is that they’ve come up with dose-dependent interpretive breakpoints:

  • MIC <0.06 ug/ml: susceptible at a dose of 5 mg/kg once a day (FYI I pretty much never use a dose that low for enrofloxacin because of concerns about efficacy and development of resistance)
  • MIC 0.12 ug/ml: susceptible at a dose of 10 mg/kg once a day
  • MIC 0.25 ug/ml: susceptible at a dose of 20 mg/kg/day
  • MIC >/=0.5 ug/ml: resistant

The lower breakpoints mean we’ll see a lot more of these bugs reported as resistant to enrofloxacin. The dose-specific guidance will help figure out when we can still use enrofloxacin to treat some of these middle-ground MIC bugs, and hopefully reduce ineffective use of this important antimicrobial.

Communicating this to veterinarians will be a challenge, since it’s a completely new concept for most.

Other breakpoint changes

Some other breakpoints have changed in addition to the reduction of the enrofloxacin breakpoints mentioned above. One is the reduction of the susceptible breakpoint for chloramphenicol and staphylococci in dogs from <8 ug/ml to <2 ug/ml. Bacteria with MICs of 4-8 ug/ml previously would have been called susceptible to chloramphenicol, but now will be reported as resistant. That’s relevant because there are a lot of staphylococci with MICs in that range, and we now consider them non-susceptible to this drug. So, we’ll see a lot more reported chloramphenicol resistance.

More species-specific breakpoints

The updated standards have also added numerous species-specific breakpoints. Unfortunately we don’t have breakpoints for all bug/drug combinations in every species. In these cases, we either have no guidance or we extrapolate from humans or other animals (better than nothing, but not ideal since there can sometimes be big differences between species). Over time, as more data become available, we eventually get to the point where we can make more specific recommendations.

Urinary breakpoints

Another subtle but important change is modification of how site-specific breakpoints are reported. Most of these are based on serum drug levels, since that’s what should be present throughout most of the body, and we tend to have the most data about serum levels. However, we know that some drugs are excreted at high levels in urine, and those high levels can make bugs that are resistant to a drug in serum susceptible to it in urine.

For that reason, there are separate serum and urine breakpoints for some drug/bug combinations (e.g. E coli and amoxicillin). Urine breakpoints are higher because more drug ends up in the urine and will be able to inhibit some bacteria that would tolerate the lower concentrations of drug in other parts of the body. Previously, this was called a “UTI” (urinary tract infection) breakpoint. The problem was urine levels are only relevant for lower urinary tract infections (cystitis), not upper urinary tract infections (e.g. pyelonephritis). For an infection in the kidney, we need to rely on the drug that’s in serum, not in urine, so the UTI terminology has lead to confusion and misreporting. Now it’s clearer as the the updated standards call those breakpoints “(lower) urinary tract (ur).“ Hopefully that will result in fewer pyelonephritis samples being interpreted with urine breakpoints and leading to suboptimal antimicrobial treatment decisions.

Labs vary in how quickly they make changes when these standards are updated. Some are on the ball right away. Some are really slow. Some never adopt all the guidelines. (I saw a lab report yesterday from the US using an old guidance that was changed 10 years ago, leading to a bad treatment choice.) Veterinarians asking labs if/when they’re going to adopt the changes may help spur them on. It’s a hassle for labs to make the changes and communicate why things are now different in the lab reports, but it’s important to get this done for obvious reasons.