Extended-spectrum- -lactamase-positive Escherichia coli mainly adds to, rather than replaces, extended-spectrum- -lactamase-negative E. coli in causing bacteraemia in Hong Kong, 2000-10

Sir, Escherichia coli producing extended-spectrum β-lactamases (ESBLs) have increased rapidly in the last 5–10 years.1,2 However, little is known about how the emergence of ESBL impacts upon the incidence rates of all E. coli infections and on infections caused by ESBL-negative strains. We assessed the trends in E. coli bacteraemia using the database of a clinical microbiology laboratory from a healthcare region in Hong Kong.3 The purpose was to examine how increasing ESBL-positive strains affect the overall burden of E. coli bacteraemia. The data on E. coli bacteraemia during an 11 year period (2000–10) was extracted, and duplicate isolates were removed by the ‘first isolate per patient’ method.3 During this period, all isolates were tested by the disc diffusion method, and ESBL production was screened using the double disc synergy test.2,4 Three proxies were used to assess changes in the burden of E. coli bacteraemia: (i) crude incidence, defined as the number of episodes per year; (ii) cumulative incidence, defined as the annual number of episodes per 10 000 hospital admissions; and (iii) incidence density, defined as the number of episodes per 100 000 patient-days. Linear regression analyses, considering potential autocorrelation in the time series data, were used to assess trend changes in the proxy measurements over time. In addition, we quantified the temporal changes by assessing the difference in the first half (period 1, 2000–05) and the second half (period 2, 2006–10) of the time period and expressed the interval change as a percentage [i.e. (period 2 − period 1) × 100/period 1]. The χ2 test was used for comparison of proportions and the Student's t-test was used to compare annual means between time periods. A P value of 0.05 was considered to indicate statistical significance. The data were analysed using SPSS version 17.0 (SPSS, Hong Kong) and R version 2.12.0 (R Development Core Team, Vienna, Austria).