a Quality Assurance Laboratory, Central Public Health Laboratory, London NW9 5HT; b Public Health and Clinical Microbiology Laboratory, Addenbrooke's Hospital, Cambridge CB2 2QW, UK
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Abstract |
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Introduction |
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Materials and methods |
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Strains were provided by the Clinical Microbiology and Public Health Laboratory, Cambridge, UK (CMPHL) or the Antibiotic Resistance Monitoring and Reference Laboratory, Colindale, UK (ARMRL), and included isolates from recent infections, isolates referred for confirmatory testing or isolates with known resistance characteristics collected from other specialist laboratories. Freeze-dried strains were distributed to participants.7
Antimicrobial susceptibility testing
MICs were determined by an agar dilution method8 in two laboratories, either the Quality Assurance Laboratory, Colindale, UK (QAL) and CMPHL, QAL and ARMRL, or CMPHL and the Antimicrobial Chemotherapy Laboratory, City Hospital NHS Trust, Birmingham, UK. Disc susceptibility testing8 and, more recently, Etests (Cambridge Diagnostic Services, Cambridge, UK) were used in QAL to confirm the susceptibility of the strains before and after freeze drying. Strains were designated as susceptible or resistant to antimicrobial agents according to the criteria of the British Society for Antimicrobial Chemotherapy (BSAC).8 The widely used three category classification was used for penicillin susceptibility of Streptococcus pneumoniae.9,10
Information supplied to participants
A report form sent with the strains reported here specified the identity of the organism and the site of isolation as other than urine. More specific details were provided with enterococci, which were said to be isolated from cases of endocarditis where combination therapy was being considered. Participants were requested to test the strains by their routine methods and designate them as susceptible or resistant (or high-level resistant in the case of enterococci tested against gentamicin) to named antibiotics. A full analysis of participants' results was provided as described previously.5
Participants
The countries participating in the scheme and the number of participants in each varied for each distribution. The current distribution of participants by country is shown in Table I. In order to reduce the risk of misleading results from small numbers of laboratories, results for individual countries are presented here only when the number of participants was 10 or more. Confidentiality of results is an important underlying principle of the scheme. The numbers of participants in each country are published in the scheme's annual report; to prevent deductive disclosure of identity results are presented here as percentages of those testing an organism achieving the designated correct result. Each country is represented by the same arbitrary letter in all the tables of results (Tables IIIX
); the letters have not been used in previous articles reporting on UK NEQAS results.
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Results |
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The seven distributed strains with reduced susceptibility to penicillin would all be classified as moderately resistant, with MICs between 0.1 and 1 mg/L. Participants' reports of resistant, moderately resistant or intermediate were regarded as correct, and of susceptible as incorrect. There were apparent differences in performance between countries, with some consistently above the mean and some consistently below (Table II). In seven countries the rate of detection of resistance for strains with MICs of 0.51 mg/L was
90%. There is no evidence of consistent improvement in performance over time.
Vancomycin resistance in enterococci
Results for five strains of enterococci with the VanB phenotype and expressing low-level vancomycin resistance are presented in Table III. Detection of such resistance appears problematical, with resistance in the most recently distributed strain being detected by a mean of 77% of participants. The percentage of correct results was consistently higher than the mean in some countries and lower in others. There were obvious improvements in detection rates over time in eight of the 10 countries.
High-level gentamicin resistance in enterococci
High-level resistance to gentamicin was reliably detected in all countries (data not shown). In contrast, eight strains that were not high-level resistant but had gentamicin MICs of 416 mg/L, which is typical of enterococci, were reported as high-level resistant by a mean of 15% of participants. There was little consistency in such reporting (Table IV), although results from some countries show a tendency to be above or below the mean success rates. There was no evidence of improvement over time.
Non-ß-lactamase-mediated resistance to ampicillin and co-amoxiclav in Haemophilus influenzae
Resistance to ampicillin in eight ß-lactamase-negative strains was not readily detected in most countries between 1985 and 1998 (Table V). Such strains are resistant to co-amoxiclav, but spurious reports of susceptibility were also common (Table VI
). Ability to detect such resistance varied between countries, with some consistently above or below the mean. There is obvious improvement over time in most countries and in the most recent distribution detection rates were 90% or above in five countries.
Chloramphenicol resistance in H. influenzae
Ability to detect chloramphenicol resistance in eight strains varied between countries with some consistently above or below the average (Table VII). There did not appear to be any improvement over time.
Methicillin resistance in staphylococci
The level of performance has been uniformly high with methicillin-resistant Staphylococcus aureus, in 1999 detection rates were 99% in every country (Table VIII
). There was more variation in performance with seven methicillin-resistant coagulase-negative strains (Table IX
), with some strains apparently being easier than others: the detection rates for three strains were 100% in seven countries. Some countries appeared to achieve consistently higher detection rates than others, and in some there was evidence of improved performance over time.
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Discussion |
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Although there were some differences between countries, the problem tests investigated generally caused difficulties in most European countries. In a previous report on performance in the UK, detection of penicillin resistance in S. pneumoniae appeared more difficult in strains with MICs of 0.10.25 mg/L than in those with MICs of 1 mg/L.6 However, this was not evident in most countries in the current study and all the strains tested presented problems in some countries. High-level resistance to vancomycin in strains of enterococci with the VanA phenotype has been detected easily in EQA specimens, but detection of low-level resistance in strains with the VanB phenotype has been problematic.11 Low-level resistance to vancomycin also caused problems throughout Europe in this study and similar difficulties have been reported in other European studies12 and in the USA.13 High-level resistance to gentamicin (MICs > 1000 mg/L) in enterococci has been detected easily in EQA specimens, but strains that are not high-level resistant have been falsely reported as resistant.11 In the current study similar errors were seen in all countries, although the mean success rate was 85%. Ampicillin- and co-amoxiclav-resistant, ß-lactamase-negative strains of H. influenzae are not reliably detected, although there does appear to be some improvement in performance over time. The level of resistance in these strains is low and the clinical consequences of such resistance remain unclear. Detection of chloramphenicol resistance in EQA samples of H. influenzae has proved difficult,14,15 and in most European countries in the current study some laboratories failed to detect this resistance. Failure rates of c. 15% in detection of resistance to methicillin in strains of S. aureus were common in early EQA distributions in the UK,6 but performance was good in all European countries in the current study. Detection of resistance in coagulase-negative staphylococci is more difficult and there are differences between species.16,17 These difficulties in testing coagulase-negative staphylococci were reflected in the results with some strains in the current study, the most recent strain distributed was difficult in that it showed a low level of resistance.
Associations have been shown between methodology and success with these more challenging organism/antimicrobial agent combinations.6 Several different methods are used in Europe and there are differences in MIC breakpoints for several agents.18 It is likely that differences in methods used in different countries have contributed to the differences in success noted and this will be investigated in the future.
Whatever the explanations for the observed differences in success rates between countries, such differences may distort comparisons of resistance rates and prevent a clear understanding of the incidence of antimicrobial resistance. Comparability of susceptibility testing results in different countries is essential for international surveillance, and international EQA has an important role to play in the demonstration and clarification of such differences.
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Acknowledgments |
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Notes |
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References |
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Hussain, Z., Stoakes, L., Massey, V., Diagre, D., Fitzgerald, V., El Sayeed, S. et al. (2000). Correlation of oxacillin MIC with mecA gene carriage in coagulase-negative staphylococci. Journal of Clinical Microbiology 38, 7524.
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Andrews, J. M., Boswell, F. J. & Wise, R. (2000). Establishing MIC breakpoints for coagulase-negative staphylococci to oxacillin. Journal of Antimicrobial Chemotherapy 45, 25961.
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Received 24 August 2000; returned 15 December 2000; revised 2 February 2001; accepted 22 February 2001