The importance of adjusting for enterococcus species when assessing the burden of vancomycin resistance: a cohort study including over 1000 cases of enterococcal bloodstream infections

The study was conducted at three different tertiary care hospitals of the Charité university hospital in Berlin, with 3011-beds in total [27]. After a confirmatory ethics vote was obtained from the Charité University Medicine ethics committee (internal processing key EA4/229/17), we performed a cohort study that included all cases of BSI caused by Enterococcus faecalis or Enterococcus faecium between January 1, 2008 and December 31, 2015. Cases were identified in the Charité microbiology database as hospitalized patients with blood cultures positive for one of these pathogens. Data on costs and hospital financial accounting was provided by the Charité Department of Financial Controlling as true hospital expenses in Euros. For all patients enrolled in this study, the following demographic and clinical characteristics were collected: age, sex, in-hospital death, length of hospital stay (LOS), day of BSI onset and stay on an intensive care unit (days). Length of stay in total and after BSI onset were defined as length of stay until death or discharge. The Charlson comorbidity index (CCI) was obtained on the basis of the patients’ diagnosed comorbidities using the method of Charlson et al. and the adaptation for the ICD-10 by Thygesen et al. [28, 29]. The original 17 Charlson comorbidity categories were cumulated based on the affected organ system in the following ten disease categories: heart disease, cerebrovascular disease, neurologic disease, lung disease, rheumatic disease, gastrointestinal disease, liver disease, diabetes, renal disease and cancer/immunological disease.

Cases were defined as patients with BSI caused by Enterococcus spp. (Enterococcus faecalis or Enterococcus faecium) during the study period. Each patient was included in the analysis once. Onset of BSI was defined as the date of the first blood culture positive for the respective pathogen. BSI was considered hospital-onset if it occurred after the third day of hospitalization. Mortality was assessed based on discharge alive or in-hospital death. Data on hospital costs were derived from true hospital costs (hospital expenses). The costs analyzed cover direct costs to the hospital of treatment and diagnostics as well as indirect hospital costs of activities without patient contact (e.g. administration, hospital maintenance). The estimated cost of individual cases was based on definite performances and on settlement keys (e.g. nurse working time per patient). Economic data was available on total hospital costs and on daily costs. A differentiation of costs before and after the infection was not available. However, as length of stay directly correlates with hospital costs, length of stay after onset for infection can be applied as proxy for infection attributable additional expenditures [2, 30, 31]. We therefore assessed the multiplicative effect on length of stay after BSI onset in a multivariable linear regression.

Descriptive, univariate analyses were performed for the total cohort, stratified by enterococcus species (i.e. vancomycin susceptible E. faecium vs. vancomycin susceptible E. faecalis) and by vancomycin susceptibility (i.e. vancomycin susceptible enterococci vs. vancomycin resistant enterococci). Since among E. faecalis-isolates only one isolate was resistant against vancomycin, only E. faecium (VSEm vs. VREm) were analyzed in the second analysis. Additionally, we compared in univariate analysis deceased patients and patients discharged alive. The median and the interquartile range (IQR) were calculated for continuous parameters; number and percentage were calculated for binary parameters. Univariate differences were tested using the Wilcoxon rank-sum test for continuous variables and the Chi-square test for binary variables.

The two linear regression analyses were performed for length of stay (LOS) after onset of enterococcal BSI by stepwise forward variable selection. The continuous parameters LOS after onset of BSI was log transformed to achieve normal distribution. Only surviving patients were included. Parameters considered in the full model were vancomycin resistance OR pathogen species (E. faecium or E. faecalis), sex, age and all underlying diseases assessed as described above. From the full model, parameters with the smallest Chi-square statistic and p > 0.05 in the type III test were removed. The regression coefficients were converted to the measures of effect using an exponential transformation and referred to as the multiplicative effect (ME) of investigated parameters. P-values < 0.05 were considered significant.

Multivariable, binary logistic regression was performed for in-hospital death by stepwise forward selection. The p-values for including a variable in the model was 0.05 and for excluding 0.06 respectively. Odds ratios (HR) with 95% confidence intervals (95% CI) were calculated. Parameters considered in the model were sex, age, pathogen species (E. faecium vs. E. faecalis) with the interaction of the vancomycin resistance and all underlying diseases assessed as described above.

All analyses were performed using SPSS (IBM SPSS statistics, Somer, NY, USA) and SAS (SAS Institute, Cary, NC, USA).

If a blood stream infection was suspected, blood cultures were drawn and incubated for up to seven days using standard blood culture tubes (BACTEC, Becton Dickinson Heidelberg Germany). If growth was detected, gram staining and culturing were performed. MALDI TOF MS and Vitek 2 automated system (Biomerieux Marcy l’etoile France) were used for identification and susceptibility testing of bacterial strains. They were interpreted using EUCAST definitions.