s.aureusinfections, 2006–2014 1227
practices are not typically in place for MSSA, which may reflect a general belief that in-hospital transmission of MSSA is rarer than MRSA transmission. After 2005, there was a decline in certain US populations
in MRSA incidence,30 including SSTIs6 and invasive MRSA infections,31,32 as a few studies showed. In this study, we examined the epidemiology of S. aureus infections, contrasting adults and children at a single center between January 2006 and March 2014. Our goal was to determine whether popu- lation strata or anatomic sites of infection drove changes in the incidence of MRSA or MSSA infections.
methods
We hypothesized that, during the study period, children, compared with adults, had a greater proportion of total S. aureus infections that were SSTIs, both for MRSA and MSSA. We further hypothesized that the incidence of MSSA infections during the study period did not change for adults or children. The Institutional Review Board of the University of Chicago Biological Sciences Division approved this study. University of Chicago Medicine is an academic medical
center with 161 pediatric and 456 adult hospital beds and an emergency department (ED) that serves 71,000 patients and outpatient clinics with 700,000 visits each year. Data were obtained from the UCM Clinical Data Research Warehouse, a repository of clinical, microbiological, and administrative data that were derived from the electronic medical record (EMR), hospital billing records, and the UCM Clinical Microbiology Laboratory. The retrospective cohort for this study included adult and pediatric patients for whom a culture obtained from any anatomic site was sent to the UCM Clinical Microbiology Laboratory and grew S. aureus between January 1, 2006, and March 31, 2014. Only the first such encounter per year was included for each patient. The cohort included those treated at UCMat 3 sites of care: as an inpatient, in an outpatient clinic, or as an ED patient. Emergency department encounters were only counted if a patient was treated in the ED and not hospitalized. Variables collected for each encounter included (1) demo-
graphics; (2) site of care; (3) dates of encounter and culture collection; (4) surgeries; (5) associated International Classifi- cation of Disease, Ninth Revision (ICD-9) codes; (6) ICD-9 code for end-stage renal disease; (7) presence of a central venous catheter; and (8) antimicrobial susceptibility data and ordering information for all cultures yielding S. aureus. For each S. aureus isolate, a clinical syndrome was classified by a single physician (M.E.A.) using data on the anatomic site of
culture from physician orders. If the syndrome was not classi- fiable from the culture order, additional information was obtained from theEMR. In total, 8 clinical syndrome categories were assigned: blood, SSTI, respiratory, joint/bone, central nervous system, urinary, other, and unknown. A 25% sample of the 1,926 cultures labeled with only the word ‘drainage’ by the ordering clinician were assessed in the EMR; all were SSTIs. Therefore, we assumed that all 1,926 cultures labeled with only
the word ‘drainage’ in the EMR culture order were from an SSTI. Infections were classified as ‘other’ if the culture was obtained from the eye, ear, sinus, or an intra-abdominal source. Isolates explicitly obtained to assess forMRSAcolonization were excluded. An infection was classified as an ‘unknown’ clinical syndrome when no available information enabled a syndrome assignment. Invasive infections were defined as isolation of MRSA or MSSA from blood, joint/bone, or central nervous system. Review of the medical record to distinguish true infec- tion from contamination or colonization was not performed. Each MRSA andMSSA infection was classified as CA, HA, or
healthcare-associated community-onset (HACO). An infection was classified as HACO when the culture was obtained in the outpatient setting or <48 hours after hospital admission from patients with 1 or more previous healthcare exposures. These exposures included surgery atUCMand/or hospital admission at UCMwithin the previous year, an ICD-9 code for end-stage renal disease (ESRD) atUCMduring the previous year, or the presence of a central vascular catheter at the time the culturewas obtained. A S. aureus infection was considered HA if the culture was obtained >48 hours after hospital admission. A S. aureus infection was considered CA if the culture was obtained in the outpatient setting or <48 hours after hospital admission from a patient without any of the above-listed healthcare exposures. Susceptibility data determined using Vitek 2 (Biomérieux,
for each quarter year, and these totals were stratified into clinical syndromes, CA, HACO, and HA S. aureus, and pediatric and adult infection categories. From administrative data, we deter- mined the total number of adult and pediatric patients treated at UCM in the ED, inpatient setting, and outpatient clinics in each quarter year. We calculated the incidence density of S. aureus, MSSA, andMRSA infections (and stratified these results into the categories noted above) in each quarter year per 1,000 patients admitted to the inpatient setting or per 1,000 patients treated in the EDor outpatient clinics, respectively. Poisson regression was used to assess for statistically significant change in incidence density in annual data. All analyses were performed in STATA version 11 software (StataCorp, College Station, TX).
results Characteristics of S. aureus Patients
In total, 5,491 MRSA (50.4%) and 5,398 MSSA isolates (49.6%) were obtained by the UCM Clinical Microbiology Laboratory during the study period from cultures not intended
Durham, NC) were obtained for each isolate for the following antibiotics: erythromycin, gentamicin, rifampin, tetracycline, ciprofloxacin, and clindamycin. We report clindamycin as resistant if the D-test for inducible clindamycin resistance was positive or based on single-agent testing because the distinction was not made in the Clinical Data Research Warehouse data. Before December 2008, the D-test was performed manually. After December 2008, the D-test was performed using Vitek. The number of MSSA and MRSA infections were tabulated
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