transmission dynamics of kpc in ltachs 1149 Therefore, we investigated the epidemiology of KPCs in 4
different LTACHs participating in a bundled KPC control intervention in the Chicago region.24 We used advanced modeling to explicitly fill in missing data (eg, missed swabs), estimate testing characteristics (eg, imperfect sensitivity of rectal cultures for KPC), determine the transmission capacity of KPC, and quantify the effect of patient cohorting on trans- mission within these 4 LTACHs.
methods Data
1 floor was to be a cohort floor where KPC-positive patients would be cared for by cohorted staff. This was done at LTACH D. For logistical reasons, this was not feasible at the other LTACHs, where the cohorting strategies were locally modified. At LTACH B, all KPC-carriers were treated in single rooms
This analysis was performed on data collected from 4 LTACHs in the Chicago region between June 11, 2012 (or June 1 in 1 LTACH), and June 30, 2013, a time within the original study period during which complete data regarding patient room occupancy were available. All patients admitted were included. During this period, a bundled intervention was implemented in all 4 LTACHs that consisted of screening all patients for KPC on admission, every-other-week point-prevalence sur- veys, bathing all LTACH patients daily with 2% chlorhexidine gluconate (CHG) cloths (Sage Products, Cary, IL), education of medical staff on KPC and infection prevention, and adherence monitoring that focused on hand hygiene.24 In addition, cohorting strategies were implemented. Ideally,
without staff cohorting. LTACHs A and C had mixed-cohort floors, ie, the majority of patients were KPC-positive but KPC- negative patients were also housed on the cohort floor and were cared for by the same staff. Previously identified KPC carriers were not screened upon readmission but were placed directly in single rooms or on a cohort floor. The study was reviewed and approved by the institutional
review board of Rush University Medical Center and granted expedited review.
Microbiology
KPC carriage status was determined from microbiological cultures of rectal swabs, obtained on admission and during point-prevalence surveys conducted every other week. These screening cultures were included in the current analysis. Sam- ples were screened using an ertapenemdisk method in a central laboratory; blaKPC was confirmed by polymerase chain reaction (PCR).25–27 In line with the study protocol, patients previously identified as KPC-positive were excluded from screening. Clinical cultures were ordered as needed by treating physi-
cians, and samples yielding carbapenem-resistant Klebsiella spp. or Escherichia coli were used in the sensitivity analysis. Clinical cultures were considered positive for KPC if a Klebsiella spp. or an E. coli isolate was isolated that displayed
intermediate susceptibility or resistance to imipenem. This approach was validated in the original analysis.24
Markov Model
A Markov model was used to describe KPC transmission. Patients were assumed to be either colonized with KPC or sus- ceptible to colonization. The rate of transition from susceptible to colonized was dependent on the number of colonized patients on the floor28 and was defined by α + β * I/N,where α is the background transmission rate, β is the patient-dependent transmission rate, I is the number of colonized patients present, and N is the total number of patients in the unit (I/N=fraction of colonized patients on the unit, or colonization pressure). In β, all transmissions were included that were dependent on the colonization pressure on the floor. This rate could include transmission from colonized to susceptible patients (either directly or through the contaminated hands of HCW) and transmission from the environment when this was dependent on the colonization pressure. All other transmissions were accounted for in α, eg, the endogenous route and transmission fromthe environment independent of the colonization pressure on the floor (including transmission from HCWs moving between floors). The endogenous route represents bacteria that were already present in the host at undetectable levels and that presumably reached detectable levels under antibiotic pressure. KPC carriers were assumed to remain colonized during their entire stay in the LTACH. We assumed that the test specificity for KPC detection was 100% and no distinction was made between different species of Enterobacteriaceae. As patients were not cultured every day and culture results
might have been falsely negative, the exact number of colo- nized patients was unknown. Therefore, a Bayesian framework using a data-augmented Markov chain Monte Carlo (MCMC) method with Metropolis-Hastings algorithm was developed, taking unobserved colonization and colonization times into account, analogous to the method used by Worby et al.29 We also estimated the probability of a patient being a KPC carrier on admission (f) and the sensitivity of the screening process (ϕ), which included the swabbing technique, transportation and storage of swabs, culture method, and accuracy of the blaKPC PCR assay (details are provided in Online Appendix A). Nurses generally were assigned to 1 floor. Therefore, every LTACH-floor was considered a distinct unit in which trans- mission could occur. The high acuity units (wards that cared for patients with higher-level medical or nursing needs) were physically separated from the general floors and had a distinct nursing team; therefore, these were considered separate units. In 2 LTACHs (B and D), 1 floor was divided into 2 separate units because they were separated both physically and in terms of nurse assignment. Each admission was considered a new admission if the patient had left the facility for at least 1 day, and the culture results obtained during previous admissions were not taken into account in the current analysis.
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