September 2010
Feature Story

Anne Paxton

Speaking before a packed conference hall at the American Society for Microbiology annual meeting in May, leading microbiologists outlined the origin of and rationale for the Clinical and Laboratory Standards Institute’s new recommended breakpoints for antimicrobial susceptibility testing, and ways they can be implemented in the laboratory. Getting these breakpoints, or interpretive criteria, right is part of the continuing need to cope with antibiotic resistance, these experts say. But how soon most laboratories will be willing or able to adopt the breakpoints remains to be seen.

In susceptibility testing of Enterobacteriaceae, the CLSI Subcommittee for Antimicrobial Susceptibility Testing changed the breakpoints for cephalosporins and aztreonam in January of this year, and those for carbapenems in June. The changes were made to improve detection of all types of resistance mechanisms to these antibiotics, says ASM speaker Janet Hindler, MCLS, MT(ASCP), senior specialist in clinical microbiology at UCLA Medical Center, Los Angeles. “There have been improvements in understanding of how the drugs work in the patient, and those played a role in the development of the new breakpoints.”

The CLSI re-evaluates the breakpoints when new data are brought to the subcommittee indicating the current ones don’t work, says fellow ASM speaker Jean B. Patel, PhD, deputy director of the Office of Antimicrobial Resistance, Centers for Disease Control and Prevention, who represents the CDC as a voting member of the subcommittee. “There could be new microbiological data indicating new resistance mechanisms or clinical outcome data, or PK/PD [pharmacokinetic/ pharmacodynamic] data suggesting the breakpoints are incorrect.” There are often new data suggesting that breakpoints established more than 25 years ago, when the drug was first approved, are no longer correct in today’s environment.

This is not the first revision to susceptibility breakpoints, says Hindler, who is a consultant to the CAP Microbiology Resource Committee. In 2006 there was a major revision for Staphylococcus aureus, and “that was in response to the emergence of vancomycin-resistant Staphylococcus. “Prior to that, there were no vancomycin-resistant strains; all staphylococci were susceptible. Then resistant strains emerged, and we realized the vancomycin breakpoints that were set before weren’t accurate.”

The cephalosporins, aztreonam, and carbapenems for Enterobacteriaceae are the focus of the 2010 breakpoint changes, Dr. Patel says. CLSI’s M23 document (Development of In Vitro Susceptibility Testing Criteria and Quality Control Parameters) outlines criteria for considering a review of the breakpoints, and the cephalosporin, aztreonam, and carbapenem breakpoints met those criteria.

In fact, they met the criteria many years ago. “These changes have been under consideration ever since the emergence of ESBL-producing Enterobacteriaceae, which was sometime in the late 1980s,” Dr. Patel says. Initially, the CLSI addressed the emergence of the ESBL (extended-spectrum β-lactamase) isolates by incorporating a phenotypic confirmatory test for ESBLs and using that test to change interpretive results for the extended-spectrum cephalosporins. “But that was really considered a kind of stopgap measure or short-term fix to the emergence of resistance in Enterobacteriaceae.”

Cephalosporins and carbapenems are two classes of antibiotics called β-lactams, the most commonly used antibiotics to treat serious infections caused by Enterobacteriaceae, Dr. Patel explains. “Usually an enterobacterium will become resistant to cephalosporins first, but still be susceptible to carbapenems. But now we’re starting to see isolates that are resistant to both. And that really limits treatment options for severe infections.”

It’s taken some time for the CLSI to present the revised breakpoints because exactly how to set them was somewhat controversial since there are limited clinical outcomes data for treatment of Enterobacteriaceae infections with cephalosporins, Dr. Patel says. “When you have older drugs like these extended-spectrum cephalosporins, you’re really relying on case reports that occur in the literature. But these data were hard to come by because labs have been instructed to report any ESBL-producing isolate as resistant to cephalosporins.” As a result, the cephalosporins weren’t being used for treatment. “That’s why most of the information used to change the breakpoints were the PK/PD analyses.”

To set the breakpoints, the CLSI subcommittee tried to look at when patients hit the correct target attainment—the point at which the antimicrobial works—and to determine the dosage where the drug activity is bactericidal as opposed to bacteriostatic, Dr. Patel says. “So that’s optimal activity; if you have 90 percent of your patients receiving dosages of drugs that are optimal, that’s where you want to set your susceptible breakpoint.” While not a hard-and-fast rule, that guideline is generally what the subcommittee looks for.

An antimicrobial susceptibility test would normally be performed in the laboratory any time an enterobacterium is isolated on a clinical specimen and is thought to be the cause of a patient’s infection, Dr. Patel notes. In many cases, treatment of the patient has already started. “Usually a patient who has an infection is going to be treated empirically by the physician, who doesn’t know what bacteria is causing the infection or the antibiotic susceptibility of that isolate, but will prescribe antibiotics, then refine the prescription depending on the lab results. The AST results could be available in two or three days or possibly longer.”

Urine cultures make up the highest volume of specimens for AST, Hindler says, but not all urinary tract infections will have a urine culture and AST ordered. “A young female with acute UTI for the first time would sometimes be treated based on symptomatology,” she notes. And in some health care facilities the provider might even take symptoms and prescribe antibiotics, such as a fluoroquinolone (for example, ciprofloxacin), over the phone. But if the UTI were treated and recurred, the physician would be more likely to do a urine culture and susceptibility testing, she says. “We’re seeing a tremendous increase in resistance to fluoroquinolones among the bacteria most commonly causing UTI.”

Breakpoints can be an issue with UTIs, but they become more complicated with hospital infections such as bloodstream or respiratory infections that could be caused by Enterobacteriaceae. “In many cases there is an unpredictable antibiotic profile, so the clinicians don’t really know what drug to use,” Hindler says. “That’s when we’ll do AST. If a culture grows a bacteria that’s likely pathogenic, probably 90 percent of the time we’ll perform susceptibility tests on that bacteria.”

A bacterial culture report will often list the Gram stain result first—whether the bacterium is a gram-positive cocci or gram-negative rod—then it will report the identification and antibiotic susceptibility profile of the bacterium, telling the clinician what drugs are resistant and what drugs are susceptible and likely to help resolve the infection.

For example, for a given bacterial isolate, a report might say “ciprofloxacin MIC=1: susceptible.” Bacterial isolates can have different MICs (minimum inhibitory concentration, defined as the “lowest concentration of an antibiotic that inhibits visible growth of a microorganism”). Since different institutions might have different populations of bacteria, “if the bacterial isolate that’s causing an infection is a species that has an unpredictable susceptibility profile, you’d want to test that isolate to determine its susceptibility to different drugs,” Hindler says.

In addition to “susceptible” and “resistant,” there is an in-between category called “intermediate.” “Depending on the body site,” Dr. Patel says, “this result means the drug might still be effective for that patient. It could also mean there’s some technical variability in the susceptibility testing, and that this isolate falls within that range, so it’s not clear if it’s truly susceptible or truly resistant.”

What will the new breakpoints mean for clinical laboratories? “The most significant thing is they are no longer being asked to perform a special test for ESBL production before reporting results for treatment,” Dr. Patel says. This will save time, but there were performance concerns with the ESBL test as well. “It was decided that those results were becoming less reliable, particularly in isolates where ESBLs were present with other β-lactam resistance mechanisms. There were also new resistance mechanisms emerging where we didn’t have good tests—for example, the AmpC-type β-lactamase enzymes, which have all the same problems that ESBL-producing isolates have.” That was one of the factors that suggested institutions needed to move away from these special tests to detect resistance, and more toward just the susceptibility result, using only that result to predict treatment outcomes, Dr. Patel says.

There is a key obstacle to widespread adoption of the breakpoints, however: a gap between the recommendations of the CLSI and those of the FDA that may persist for some time. “The FDA set its breakpoints many years ago, in fact when the drugs first became available,” Hindler says. The regulatory mechanism by which the FDA is able to modify drug labels can move slowly in making changes because it involves input from the companies responsible for the drugs. The FDA has started the process by issuing guidance (www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/ Guidances/UCM169359.pdf) and notifying drug manufacturers of the need to review their drug package inserts and revise them with FDA approval to meet current drug testing criteria.

Laboratories could implement the new CLSI breakpoints right away by doing an in-house verification of the susceptibility testing method, Dr. Patel points out, and some labs have done so. “But other labs are deciding to wait until the FDA adopts these breakpoint changes, and it’s hard to say when that’s going to happen.” Either the FDA or CLSI breakpoints are acceptable to clinical laboratory accrediting bodies such as the CAP.

As required by the Food and Drug Administration Amendments Act of 2007, the FDA issued the guidance document in 2009 for industry to update labeling of susceptibility test information in systemic antibacterial drug products and AST devices. However, the appropriate role of the CLSI’s standard-setting has not been resolved. “The diagnostic manufacturers have to have their kits FDA-cleared before we can use them on patient isolates, and around 80 percent of clinical laboratories use an FDA-cleared AST system. But the tests for AST that are FDA-cleared have to use FDA breakpoints. The manufacturers are not at liberty to use CLSI breakpoints, and that’s the big dilemma,” Hindler says.

“So the labs are stuck. The diagnostic manufacturers are stuck.” However, because under CLIA regulations tests have to be verified in labs even if they are FDA-cleared, “we’re saying labs can verify the ASTs in their own laboratory with the new breakpoints,” she says. The revised breakpoints can be used providing: 1) the AST system contains the lower antimicrobial concentrations needed to accommodate the revised breakpoints; 2) there is a mechanism, either in the system software or manually, to interpret MIC results using the revised breakpoints; and 3) an in-house verification is performed.

In reality, though, Hindler does not think most laboratories are going to adopt the new breakpoints right now. “Most are going to wait a year or more, however long it takes for the FDA to change the breakpoints so the manufacturers can change their breakpoints. It’s not difficult for a lab to verify, but it’s an effort for the lab because the manufacturer hasn’t been able to update the breakpoints.” As antibiotic resistance continues to worsen in the United States, this is making it more complicated for labs to be able to detect resistance, she says.

Hindler first became interested in antibiotics in 1977—when these issues didn’t exist. “The antibiotics seemed to be working back then.” But in the 1980s, resistance started appearing with the emergence of MRSA (methicillin-resistant Staphylococcus aureus), and a whole series of problems has followed. “The drugs aren’t able to keep up with the resistance mechanisms, and the pharmaceutical companies aren’t that interested in developing new antibiotics, so there’s an increasing number of infections that can’t be effectively dealt with.”

Treatment options for some bacterial infections can be extremely limited. Dr. Patel says: “In this country we’re experiencing Enterobacteriaceae isolates that are resistant to almost all antibiotics that would be considered for treatment, and there have been reports of ‘pan-resistant’ isolates—organisms that are resistant to every drug you would consider for treatment. For that reason, it’s important to have the most accurate susceptibility test results that we can produce, and that’s one of the goals behind the breakpoint changes.”

By having more accurate susceptibility breakpoints, laboratories can avoid overcalling resistance, Dr. Patel points out. Overcalling resistance potentially makes resistance worse because it drives the unnecessary use of broader-spectrum drugs.

Given the overall framework of increasing resistance, is it increasingly important to get breakpoints exactly right? That is certainly true with the carbapenems, says Dr. Patel. “The breakpoint changes will really help in identifying isolates with unusual resistance. More isolates with these resistance mechanisms will test nonsusceptible, and that’s an easy flag to tell laboratories to look for.”

Drug dosing and drug administration are factors that affect target attainment, Dr. Patel says. “Most of the studies looking at alternative drug administration routes (such as continuous infusion versus intermittent) are happening with the carbapenems, because those are drugs where we’re really trying to preserve their activity. For some of these isolates that have multiple resistance mechanisms, those drugs might be your only option—even though the MIC is higher than the clinician would like.”

But determining breakpoints is not an exact science, Dr. Patel cautions. “When people raise this concern about cefepime, there’s no definitive data to say the breakpoints should definitely be lower or are definitely okay where they are. The only thing that would help resolve the questions would be more clinical outcome data.” But those studies are expensive and hard to do once a drug is approved. “Once that’s done, the only data you can usually get are single case reports.”

A 2007 study of clinical outcomes of cefepime shows that when the MIC is higher, the mortality rate is higher. But there has been controversy about whether the breakpoint should be set at MICs of 8 or less for the isolate to be called susceptible. “People have argued that the breakpoint should be lower,” Dr. Patel says. “The problem is a lot of these patients were not treated with the same dosages at which the breakpoints were set. So it could be if you treated them at a higher dosage of cefepime, you’d get better outcome data here. You can say that might be the case, but we don’t have that data.” For cefepime, the CLSI breakpoints have not changed and thus they remain the same as the FDA breakpoints, she says. The antimicrobials that are different are aztreonam, ceftriaxone/cefotaxime, ceftazidime, and cefazolin, which happen to be some of the most commonly used cephalosporins.

While breakpoints are primarily used for guiding treatment, they are critical for another reason: They help decide where infection control precautions need to be implemented. “If you had a patient infected with a resistant bacteria you were worried about spreading in your hospital, you would place that patient under infection control precautions, so visitors would need to wear a gown and gloves,” Dr. Patel says. But the choice has cost implications and can have an adverse psychological impact on patients. “There are a lot of reasons for trying to limit it,” she says.

A number of institutions have been using special tests such as the ESBL test or the modified Hodge test to identify patients with infections that need infection control precautions. But Dr. Patel believes with the new breakpoints, labs don’t have to do these tests either to guide treatment or to make infection control decisions. “You can really use your susceptibility results to guide your infection control decisions. If institutions just rely on the routine susceptibility report without having to perform any special tests, it gives them the benefit of simplicity and speed.”

Antibiotic resistance promises to worsen in the U.S. and globally. According to a new Lancet Infectious Diseases article (see Another resistance mechanism poses worldwide threat), drug development programs appear to be “insufficient to provide therapeutic cover” against gram-negative bacteria in 10 to 20 years. Although groups like the Infectious Diseases Society of America have programs to encourage more governmental attention to all antibiotic resistance, “there will probably have to be a crisis before somebody sits up and really takes notice,” Hindler says. But she and Dr. Patel emphasize that the new antimicrobial susceptibility breakpoints significantly improve a test that is important both for treating individual patients and for controlling the spread of infectious diseases.

Related Links

• MIC breakpoints
  (PDF, 21 KB)
• Another resistance mechanism poses worldwide threat