Effects of duplicate and screening isolates on surveillance of community and hospital antibiotic resistance

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Abstract
Objectives: To investigate common contentions that duplicate and screening isolates consistently show marked excess resistance, and that inclusion of such isolates significantly distorts regional resistance estimates. Methods: Two Welsh surveys of antibiotic resistance for routine diagnostic isolates were analysed, comprising 309 129 isolates of six common community pathogens and 85 061 ward isolates of 11 common hospital pathogens. Duplicate isolates were defined as isolates from the same patient of the same pathogen with an indistinguishable susceptibility pattern, excluding the initial isolate. Significance was assessed from 95% confidence limits of the difference between resistance estimates. Results: Duplicate isolates comprised ∼20% of total isolates. For the 195 antibiotic–pathogen combinations investigated, differences in resistance between duplicate and non-duplicate isolates were statistically significant for 93. Only 54 combinations showed significantly increased resistance amongst duplicates, and only 30 of these showed a difference >5%. Comparisons of de-duplicated with un-de-duplicated regional resistance estimates showed significant differences for only 18 of 195 antibiotic–pathogen combinations; none were sufficient to alter judgement on clinical use. Screening isolates produced little disturbance of resistance estimates for Staphylococcus aureus, with the exception of flucloxacillin resistance, where inclusion of screening and duplicate isolates resulted in an increase of 4.4% for both community and hospital resistance estimates. Conclusions: The contentions were incorrect for these regional surveys. However, the proportion (and so effects) of screening and duplicate isolates may be greater in surveys of units with frequent repetitive sampling practice (burns, ITU, cystic fibrosis), or pathogens subjected to unusually intensive infection control sampling.

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