Department of Infection, Guy's, King's and St Thomas' School of Medicine, St Thomas' Hospital, London SE1 7EH, UK
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Abstract |
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Materials and methods: Trends in resistance were analysed for recent isolates of selected organisms from Guy's and St Thomas' Hospitals with the use of various criteria for the exclusion of duplicates, including time since the last isolate and antibiogram pattern, and the effect of excluding screening specimens.
Results: There was a significant difference of about 8% in the apparent frequency of methicillin resistance in Staphylococcus aureus in inpatients if the time limit for duplicates was set at 5 rather than 30 days; it was about 10% if a 5 day limit was compared with a 365 day limit. There was also a significant difference, of 610%, in apparent resistance frequencies if isolates from screening specimens were excluded. Apparent gentamicin resistance rates in Klebsiella spp. varied between 11% and 28%, and the number of apparent patient isolates of gentamicinresistant organisms varied by up to 35%, depending on the duplicate exclusion criteria chosen. Effects were smaller, though still significant, for vancomycin resistance in Enterococcus spp. There was little effect for amoxicillin or cefuroxime resistance in Escherichia coli isolates from general practitioners, where the proportion of duplicates was small.
Conclusion: Improved surveillance of antibiotic resistance is needed. However, care needs to be taken in setting the criteria for classifying isolates as duplicates and in comparing results where these criteria may be different or unknown.
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Introduction |
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An automated system3 that gathers information from a large number of hospitals in the USA has a limit of 5 days, after which repeat isolates are not considered duplicates. A report on antibiotic resistance in organisms from blood cultures excluded similar organisms isolated within 7 days.4 In the past, we have excluded repeat isolates within a calendar year, but did not state this clearly.5 In many situations it will probably make little difference to resistance frequencies whether or not repeat isolates are excluded and what the definition of a repeat isolate is. However, in some situations substantial effects are possible.
Many resistant organisms, such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE) and gentamicin-resistant klebsiellae, often cause colonization rather than infection. Colonization may persist for months or years.68 Repeated admissions and screening of colonized patients adds greatly to the likelihood of duplicate isolates of resistant strains in surveys of laboratory results.
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Materials and methods |
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Ninety-five per cent confidence intervals (CIs) of resistance frequencies were calculated by the binomial method.9
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Results and discussion |
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Figure 2 shows results for gentamicin resistance in Klebsiella. There were significant differences in apparent resistance frequencies when results with the upper time limit for duplicates set at 5 days were compared with 30 days. Exclusion of screening specimens resulted in a significant reduction in apparent resistance frequencies. Figure 2
also shows the effect of using antibiograms to help identify duplicates. As has been noted previously,10 MIC breakpoints that fall in troughs of bimodal or polymodal MIC distributions are most likely to yield reproducible classifications as susceptible or resistant. For example, MIC distributions of co-amoxiclav or cefuroxime for isolates of Klebsiella with decreased susceptibilities straddle the breakpoint for resistance, and therefore reproducibility of classification of multiple isolates of this type from a single patient is likely to be less than perfect. This prediction is supported by the finding that if antibiograms were ignored, the percentage resistance to gentamicin was often significantly lower than if differences in the results for one or more of the compounds always tested against Klebsiella disqualified an isolate from designation as a duplicate. There was a smaller effect if results for gentamicin alone were considered. Thus Figure 2
shows that gentamicin resistance rates in Klebsiella varied from 27.7% to 11.4%, accompanied by a reduction of 35% in the number of patient isolates, depending upon the exclusion criteria.
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Improved surveillance of antibiotic resistance is needed. However, care needs to be taken in setting the criteria for classifying isolates as duplicates and in comparing results where these criteria may be different or unknown. Several resistant organisms that are currently recognized as causing problems in hospital patients, including MRSA, glycopeptide-resistant enterococci and multiply resistant klebsiellae, persist in patients for long periods. Furthermore, the patients may remain in hospital for long periods, or require treatments (e.g. renal dialysis) that require frequent (re-)admission to hospital. Therefore, we believe that 365 days is the best time period to use when classifying isolates as duplicates.
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Notes |
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References |
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2 . House of Lords Select Committee on Science and Technology. (1998) Seventh Report: Resistance to Antibiotics and Other Antimicrobial Agents. Stationery Office, London, UK.
3 . Sahm, D. F., Marsilio, M. K. & Piazza, G. (1999). Antimicrobial resistance in key bloodstream isolates: electronic surveillance with The Surveillance Network DatabaseUSA. Clinical Infectious Diseases 29, 25963. [ISI][Medline]
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5 . Phillips, I., King, A. & Shannon, K. (1986). Prevalence and mechanisms of aminoglycoside resistance. A ten-year study. American Journal of Medicine 80, 4855. [ISI][Medline]
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7 . Mulligan, M. E., Murray-Leisure, K. A., Ribner, B. S., Standiford, H. C., John, J. F., Korvick, J. A. et al. (1993). Methicillin-resistant Staphylococcus aureus: a consensus review of the microbiology, pathogenesis, and epidemiology with implications for prevention and management. American Journal of Medicine 94, 31328. [ISI][Medline]
8 . Hart, C. A. & Gibson, M. F. (1982). Comparative epidemiology of gentamicin-resistant enterobacteria: persistence of carriage and infection. Journal of Clinical Pathology 35, 4527. [Abstract]
9 . Armitage, P. & Berry, G. (1987). Statistical Methods in Medical Research, 2nd edn. Blackwell, Oxford.
10 . Working Party on Antibiotic Sensitivity Testing of the British Society for Antimicrobial Chemotherapy. (1991). A guide to sensitivity testing. Journal of Antimicrobial Chemotherapy 27, Suppl. D, 148. [ISI][Medline]
Received 20 July 2001; returned 13 October 2001; revised 19 October 2001; accepted 22 October 2001