JAC Advance Access originally published online on March 1, 2007
Journal of Antimicrobial Chemotherapy 2007 59(5):913-918; doi:10.1093/jac/dkm040
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Effect of excluding duplicate isolates of Escherichia coli and Staphylococcus aureus in a 14 year consecutive database
1 Department of Clinical Microbiology, Central Hospital, Varendsgatan 8, Växjö, Sweden 2 Department of Medical Sciences, Uppsala University Hospital, Uppsala, Sweden
Received 15 November 2006; returned 12 December 2006; revised 20 January 2007; accepted 29 January 2007
* Corresponding author. Tel: +46-470-587478; Fax: +46-470-587455; E-mail: martin.sundqvist{at}ltkronoberg.se
Objectives: It is recommended that duplicate isolates are excluded when reporting resistance rates. The rationale for this is that failing to do so will yield falsely high resistance rates. We analysed a 14 year consecutive database of Escherichia coli (n = 62 380) and Staphylococcus aureus (n = 28 178) using various cut-off algorithms to determine the importance of excluding duplicates and principal differences between the bacteria.
Methods: Susceptibility testing was performed according to the Swedish Reference Group for Antibiotics guidelines. Duplicates were excluded on the basis of species, individual and time (exclusion cut-offs of 7, 14, 30, 45, 90, 180, 270 and 365 days) from the first isolate.
Results: Although 30% of the isolates were excluded using a 365 day exclusion algorithm, the effects on resistance rates of excluding duplicates were small. Irrespective of cut-off, resistance in S. aureus decreased when duplicates were excluded. Using 7–30 days cut-offs, resistance in E. coli decreased or was not affected, whereas higher resistance rates were obtained when exclusion was based on a 365 day cut-off. Fluoroquinolone resistance was a clear exception to this rule.
Conclusions: Although the effect of exclusion of duplicates was minor, we suggest that exclusion cut-offs should match the study timeline. The data presented on E. coli, from urinary tract infections, and S. aureus, from skin and soft tissue infections, suggest that E. coli infection, >90 days after the first culture, is mainly caused by new less-resistant strains. Patients with S. aureus continue to be colonized with the same strain.
Keywords: antimicrobial resistance , surveillance , urinary tract infections , skin and soft tissue infections
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