Data collection and definitions
We performed a retrospective study in IRCCS AOU San Martino – IST, a 1,300-beds tertiary adult acute-care teaching hospital in Genoa, Italy. Between 1 January 2007 and 31 December 2014, numbers of hospital patient-days were obtained from the digital archives of patients’ clinical charts. Then, numbers of both carbapenem-susceptible (C-S) BSI and C-R Kp BSI were identified through the computerized microbiology laboratory database. The completeness and quality of laboratory data was successfully assessed by matching the hospital admission code with data obtained by clinical charts.
Only health-care associated C-R Kp BSI were considered for the analysis. According to the European Center for Disease Control and prevention (ECDC) definitions, a health-care associated C-R Kp BSI was defined as a positive blood culture collected at least 48 h after hospital admission, or within 48 h from hospital admission in those patients who had been discharged in the preceding two days [12]. For patients with multiple episodes of C-R Kp BSI, a novel event was considered as independent if occurring at least 30 days after the last positive blood culture [12].
This epidemiological analysis was performed within the Institutional surveillance of C-R Kp BSI that is periodically reported to the Regional Health Authority as a component of the Regional Plan for the Healthcare-associated infections Prevention and Control, approved by regional and national laws [13, 14]. The study involved the analysis of existing anonymized clinical and laboratory data. An informed consent for the use of anonymized data for scientific purposes is signed by all patients admitted to IRCCS AOU San Martino – IST and included in surveillance databases. The study has been approved by the Regional Ethics Committee of Liguria Region.
Microbiology
The Vitek 2 system (bioMérieux, Marcy l’Etoile, France) was used for Kp identification and antimicrobial susceptibility testing. The interpretative breakpoints were based on the European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria (EUCAST breakpoint tables for interpretation of MICs and zone diameters, version 4.0, 2014; http://www.eucast.org). For the analysis of data, Kp isolates which were resistant to one or more carbapenems tested in our institution (i.e., ertapenem, imipenem, or meropenem) were considered to be C-R, while isolates showing full or intermediate susceptibility to all tested carbapenems were classified as C-S.
Statistical analysis
The primary study analysis aimed to establish the annual incidences of C-R Kp BSI during the study period. Annual incidences of C-R Kp BSI with their 95 % confidence intervals (CI) were calculated as the number of events per 10,000 patient-days. In addition, annual incidences of C-R Kp BSI were also stratified by subgroups according to the ward where the diagnosis of C-R Kp BSI was made (i.e., intensive care units [ICUs], medical wards, surgical wards, or rehabilitation wards). A Chi square test for linear trend was used to assess the change in the incidence of C-R Kp BSI in our hospital over the study period.
An additional aim was to detail the overall trends in the incidence of Kp BSI. Therefore, annual incidences of health-care associated C-S Kp BSI were also calculated, by means of the methods described above. Finally, crude 30-day mortality rates were assessed both for C-R Kp BSI and for C-S Kp BSI.
All the analyses were performed using Epi-Info 7.0 (Centers for Disease Control and Prevention, CDC, Atlanta, GA, USA) and the SPSS Statistics version 20.0 (IBM Corp., Armonk, NY, USA).