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Keywords:

  • breakpoints;
  • cephalosporins;
  • CLSI;
  • Enterobacteriacae;
  • EUCAST;
  • review

Abstract

  1. Top of page
  2. Abstract
  3. Breakpoints need revision
  4. The revised cephalosporin breakpoints
  5. References

It has long been acknowledged that the cephalosporin breakpoints used in most European countries and the USA fail to detect many or most extended spectrum β-lactamases (ESBLs) in Enterobacteriaceae and that all ESBLs are clinically significant. Therefore, microbiological laboratories have undertaken not only regular cephalosporin susceptibility tests based on breakpoints, but also special tests to detect all ESBLs. An increasing accumulation of clinical data implies that the clinical success of third generation cephalosporin therapy is related more to the minimum inhibitory concentration (MIC) than to the presence or absence of an ESBL. However, the breakpoints must be lower than those previously recommended by many breakpoint committees. In Europe, this adjustment has been achieved by EUCAST (European Committee on Antimicrobial Susceptibility Testing) through the ongoing process of harmonising European breakpoints. In the USA, the CLSI recently voted to adopt similar guidelines but are waiting to implement these while revising other β-lactam breakpoints. As Enterobacteriaceae are becoming increasingly resistant, a less ‘diehard’ interpretation of the relationship among MICs, ESBLs and clinical outcome may provide therapeutic alternatives in difficult situations.

Both the European Committee on Antimicrobial Susceptibility Testing (EUCAST) [1] and the Clinical Laboratory Standards Institute (CLSI) [2] in the USA have recently revised breakpoints for third-generation cephalosporins. In Europe, this was part of the ongoing European harmonisation of clinical breakpoints for all existing antimicrobials and involves the national breakpoint committees in Europe, including the CA-SFM [3] in France, the DIN [4] in Germany, the CRG [5] in The Netherlands, the NWGA [6] in Norway, the SRGA [7] in Sweden, and the BSAC Working Party on Antimicrobial Susceptibility Testing [8] in the UK. EUCAST is organised through the European Society for Clinical Microbiology and Infectious Diseases (ESCMID) [9] and is financed by ESCMID, the national breakpoint committees and a 3-year (2005–2007) grant from DG Sanco of the European Union with a 1-year extension through the European Centre for Disease Control.

The new European cephalosporin breakpoints were finalised on 31 March 2006 (alongside those for aztreonam and carbapenems). The cephalosporins for intravenous use that were dealt with were cefuroxime, cefotaxime, ceftriaxone, ceftazidime and cefepime. In the USA, the CLSI Working Group on Enterobacteriaceae made presentations to the CLSI, and a vote was called for, during several meetings (2003–2005). However, new CLSI breakpoints will not be operative until CLSI procedures for revising breakpoints, and their legal implications, have been resolved. The breakpoints recommended by CLSI and by national breakpoint committees in Europe prior to the revision are shown in Table 1.

Table 1.   Cephalosporin breakpoints prior to revision
Breakpoint committeeaCountryCefuroxime S≤/R>Cefotaxime S≤/R>Ceftriaxone S≤/R>Ceftazidime S≤/R>Cefepime S≤/R>
  1. aSee text for abbreviations.

  2. NA, not available.

  3. All breakpoints, including CLSI breakpoints, are expressed as X/Y, interpreted as S ≤ X, R > Y.

BSACUK8/161/11/12/21/1
CA-SFMFrance8/324/324/324/324/32
CLSIUSA8/168/328/328/168/16
CRGThe Netherlands4/164/164/164/16NA
DINGermany4/82/84/164/164/16
EUCASTEurope
NWGANorway0.5/81/41/41/8NA
SRGASweden8/80.5/10.5/12/40.5/1

A major issue in both committees has been whether new clinical Enterobacteriaceae MIC breakpoints could predict clinical success and failure even without ancillary tests or whether laboratories need to continue to screen for, and confirm the presence or absence of, extended spectrum β-lactamases (ESBLs) before issuing a susceptibility report. Having reviewed the available data, both EUCAST and the CLSI concluded that: (i) there was a need for lowering of many of the current breakpoints; and (ii) correct clinical breakpoints could obviate the need for ESBL screening for the prediction of clinical outcome, whereas both detection and characterisation would continue to be of importance for infection control and surveillance purposes.

Breakpoints need revision

  1. Top of page
  2. Abstract
  3. Breakpoints need revision
  4. The revised cephalosporin breakpoints
  5. References

The history of breakpoints has shown that initial breakpoints are often overly optimistic. Almost without exception, revisions have resulted in a lowering of the initial breakpoint. New resistance mechanisms need to be assessed, doses and indications may change, and new drugs within the class provoke a need for re-evaluation of the breakpoints of existing drugs. As tools for determining breakpoints improve, older breakpoints can be subjected to re-examination using the new tools.

The proliferation of cephalosporin breakpoints in Europe highlighted the need for revision and harmonisation in itself. Some committees felt that their existing breakpoints did not allow detection of important resistance mechanisms and did not correlate well with clinical outcome. Recent studies and compilations of clinical data suggest that clinical outcome is better correlated with the MIC value than with the presence or absence of an ESBL enzyme [10–13]. Furthermore, the screening techniques used in addition to, or as a substitute for, relevant MIC breakpoints need constant adjustments to keep up with the rapidly increasing number of β-lactamases in a rapidly rising number of species [14]. Laboratories are currently required to make sure: (i) that the MIC is equal to or below (or the equivalent zone diameter is equal to or above) the breakpoint; and (ii) that the isolate is devoid of an ESBL. Thus, despite the fact that the isolate may be characterised as susceptible according to the breakpoint, the laboratory needs to exclude the presence of a resistance mechanism. When a resistance mechanism is detected (by use of a screening test), the laboratory needs to determine whether this is an ESBL, in which case the isolate should be automatically reported as resistant to that and other cephalosporins and to penicillins and aztreonam, even though the efficacy of β-lactamase inhibitors is debated. If the ESBL test is negative, a different resistance mechanism (e.g., AmpC, impermeability) is assumed, in which case the isolate may be categorised as susceptible to that cephalosporin. However, a separate test would be required in order to report on other cephalosporins. It is not surprising that the screening for and identification of ESBLs often delay the susceptibility report by one or more days and that many laboratories find it difficult to keep up with changing and complicated recommendations. The expensive, time-consuming and no less complicated alternative is to subject each isolate to a wide range of tests upon initial evaluation.

Considering all these points, cephalosporin breakpoints were subjected to independent revision by the EUCAST and the CLSI, using modern tools such as pharmacokinetic and pharmacodynamic considerations [15], modern dosing (Table 2) and the results from several compilations of clinical outcome data, all indicating that the MIC value was the important factor in predicting clinical outcome [10–13].

Table 2.   Dosages of third-generation cephalosporins relevant for EUCAST revised cephalosporin breakpoints
 Daily dosage
LowHighMaximum (g)
Cefuroxime0.75 g × 31.5 g × 34.5
Cefotaxime1 g × 3≥2 g × 312
Ceftriaxone1 g × 1≥2 g × 14
Ceftazidime1 g × 32 g × 36
Cefepime1 g × 32 g × 36

The revised cephalosporin breakpoints

  1. Top of page
  2. Abstract
  3. Breakpoints need revision
  4. The revised cephalosporin breakpoints
  5. References

The revised cephalosporin breakpoints are listed in Table 3 and the doses used by EUCAST in setting the new breakpoints are listed in Table 2.

Table 3.   Cephalosporin breakpoints following recent EUCAST and CLSI revisions
Breakpoint committeeLocationCefuroxime S≤/R>Cefotaxime S≤/R>Ceftriaxone S≤/R>Ceftazidime S≤/R>Cefepime S≤/R>
  1. aThe S breakpoint for cefuroxime was adjusted—the pharmacokinetics and pharmacodynamics suggest a breakpoint of S ≤4 mg/L. To avoid dividing the cefuroxime MIC distributions of wild-type Enterobacteriaceae (Fig. 1), both committees increased the S breakpoint to 8 mg/L and suggested that the higher cefuroxime dosage be used for infections with Enterobacteriaceae (Table 2).

  2. bThe ceftazidime and cefepime S breakpoints were adjusted from 4 to 1 mg/L to ensure that Enterobacteriaceae with clinically important extended spectrum β-lactamases (ESBLs) were not reported as susceptible.

  3. cCLSI breakpoints will not be operative until other β-lactam breakpoints have also been revised.

  4. dEUCAST breakpoints (31 March 2006) will be implemented during 2007 by national breakpoint committees in Europe.

  5. eEUCAST pharmacokinetic/pharmacodynamic breakpoints—as part of the EUCAST breakpoint process, EUCAST determines the theoretical breakpoint for each antimicrobial agent. This is based primarily on the pharmacokinetic and pharmacodynamic properties of the drug.

  6. All breakpoints were expressed as S ≤ X/R > Y.

CLSIcUSA8a/81/21/24/88/16
EUCASTdEurope8a/81/21/21/8b1/8b
EUCAST PK/PDe4/81/21/24/84/8

Neither committee intended for the revised breakpoints to detect all ESBL-producing isolates of Enterobacteriaceae. The breakpoints were determined as clinical breakpoints, i.e., to predict clinical outcome. However, in the majority of cases, and in comparison with the majority of the hitherto recommended breakpoints, the new breakpoints will allow detection of isolates with ESBLs [16].

Both committees recommend that, for epidemiological reasons, laboratories should continue to characterise resistance to third-generation cephalosporins. Correct species identification, detection and characterisation of resistance mechanisms and, above all, the typing of isolates have obvious roles in infection control and resistance surveillance. In addition, with the rising incidence of ESBL-producing Enterobacteriaceae in and outside hospitals, it becomes increasingly important to submit all Enterobacteriaceae isolates, not only those from cases of septicaemia, to cephalosporin susceptibility testing, and to devise alert systems that will indicate clonal and polyclonal outbreaks of ESBL-producing Enterobacteriaceae in hospitals and the community.

The revised EUCAST cephalosporin breakpoints should ensure a clinically meaningful cephalosporin susceptibility categorisation of Enterobacteriaceae. The frequent delay in reporting can be eliminated by recommending breakpoints that provide susceptibility categorisation without additional tests. If this is not the case, the breakpoint committees have failed.

In daily practice, the revised breakpoints will mean that if any of cefotaxime, ceftriaxone, ceftazidime or cefepime tests ‘R’, and no other cephalosporin tests were performed, the laboratory must report the tested cephalosporin as ‘R’, with a warning about the probability of other cephalosporins testing resistant. The same would apply, in the absence of other information, to a positive cefpodoxime ESBL screen test but, compared with a combination of cefotaxime and ceftazidime, this has a substantially higher percentage of ‘false-positives’ [17].

The issue that is now generating discussion is whether an isolate testing ‘S’ for one third-generation cephalosporin and ‘R’ for another (e.g., cefotaxime being ‘R’ and ceftazidime ‘S’) can be reported as tested, with (or even without) a warning about the possibility of an ESBL. The studies referred to earlier suggest that this is the case. Other investigators insist that there is enough evidence to the contrary. The controversy is difficult to resolve. To conduct a prospective clinical study would be difficult, and most available clinical evidence is anecdotal and/or generated with the high breakpoints and ESBL-screening strategies recommended by the CLSI. The discussion will go on for some time to come.

The EUCAST epidemiological cut-off values (Table 4, Fig. 1) offer an alternative to using cefpodoxime for sensitive screening for (and quantitation of) ESBLs in Enterobacteriaceae. Any isolate found to be outside the non-wild-type for either cefotaxime and/or ceftazidime and/or cefepime (Fig. 1) should be suspected of producing an ESBL and subjected to further analysis. Techniques to confirm and characterise ESBLs and other broad-spectrum β-lactamases are described elsewhere [18, internal reference].

Table 4.   Enterobacteriaceae epidemiological cut-off values (wild-type (WT) ≤ X mg/L) for cephalosporins (see http://www.eucast.org)
 Escherichia coli WT≤ (mg/L)Klebsiella pneumoniae WT≤ (mg/L)Klebsiella oxytoca WT≤ (mg/L)Proteus mirabilis WT≤ (mg/L)Citrobacter freundii WT≤ (mg/L) Enterobacter spp. WT≤ (mg/L)Salmonella spp. WT≤ (mg/L)
  1. NA, not available.

  2. Isolates with WT MIC values should be devoid of extended spectrum β-lactamases (ESBLs) or other resistance mechanisms.

Cefuroxime888488–1616
Cefotaxime0.250.120.120.060.50.50.5
Ceftriaxone0.250.120.120.06NA0.5NA
Ceftazidime0.50.50.50.12112
Cefepime0.120.120.120.120.120.12NA
image

Figure 1.  MIC distributions for wild-type Escherichia coli and Klebsiella pneumoniae for cefotaxime (top), ceftazidime (middle) and cefepime (bottom) from the EUCAST website (http://www.eucast.org), last accessed on 28 December 2006. Epidemiological cut-off values are shown in the lower left corner and the clinical breakpoints in the lower right corner.

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To perform susceptibility testing and screening for ESBLs simultaneously, the revised EUCAST clinical breakpoints, in combination with the epidemiological cut-off values, can be used. To test Escherichia coli and Klebsiella pneumoniae (and, when relevant, Proteus mirabilis, Citrobacter spp. and Salmonella spp.), cefotaxime (or ceftriaxone) and ceftazidime should be used. The results, MIC values and inhibition zone diameters can be interpreted according to the clinical breakpoint and the epidemiological cut-off for each drug. The breakpoint will give the clinical susceptibility categorisation for the two cephalosporins, and the epidemiological cut-off will disclose the possible presence of an ESBL (or other resistance mechanisms). This provides, within 18–20 h, a clinical susceptibility report and a screen for ESBLs and other third-generation resistance mechanisms and allows these isolates to be subjected to further characterisation of the resistance mechanism.

In summary, an isolate with an MIC value above (or zone diameter correlate below) the epidemiological cut-off for that species should be suspected of having a resistance mechanism, which may be an ESBL. It is still controversial whether it is safe to classify isolates with MIC values below (or zone diameter correlates above) the revised EUCAST (or CLSI) clinical breakpoint as susceptible to the drug in question unless a specific ESBL-screening test has been performed. Old habits die hard, and many microbiologists will hesitate to report an E. coli or K. pneumoniae isolate as susceptible to a cephalosporin once an ESBL has been detected, even though studies show that failures are associated with cefotaxime, ceftriaxone or ceftazidime MICs of 4 mg/L or more.[12] Lowering the susceptiblity breakpoints of cefotaxime, cetriaxone, ceftazidime and cefepime to 1 mg/L should provide a wider margin of safety for those who wish to report cephalosporin susceptibilities in Enterobacteriaceae as tested. For epidemiological reasons, the revised breakpoints should be combined with screening techniques to detect ESBLs or other broad-spectrum β-lactamases [18]. However, susceptibility categorisation (S, I and R) must not be delayed by a desire to confirm and/or characterise resistance mechanisms.

References

  1. Top of page
  2. Abstract
  3. Breakpoints need revision
  4. The revised cephalosporin breakpoints
  5. References
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