1152 infection control & hospital epidemiology october 2015, vol. 36, no. 10


every-other-week surveillance swabs, hand hygiene, the quantity of CHG cloths ordered per patient per month, or patient-related variables (eg, length of stay, age, gender) were not significantly associated with the number of acquisitions (Online Appendix B).


Sensitivity Analysis


When positive clinical cultures for KPC were added, parameter estimates remained largely unchanged (Online Appendix C, Table S1). When (artificial) positive cultures for previously known carriers were added (Online Appendix C, Table S2), the estimates for the transmission parameters were lower, whereas the probability to be positive on admission was higher than in the main analysis.


discussion


The results of this study show that in 4 LTACHs with high endemic prevalence of KPC, the per admission reproduction number, RA, was 0.40. This value indicates that patient-to- patient transmission of KPC during a single admission is not enough to maintain endemicity in a setting with daily chlor- hexidine bathing, staff education, and cohorting. Admissions of colonized patients and endogenous selection conserve the transmission cycle of KPC. These findings strongly suggest that the admission of colonized patients is a main driver of the KPC epidemic in these LTACHs. In the current study, different units were regarded as sepa-


rate entities, but spillover between units was likely. For example, nurses may have taken over shifts on other floors, and other staff, such as doctors and physiotherapists, visited patients on all units and may have contributed to the spread of KPC. This possibility was captured in the parameter α, toge- ther with endogenous selection. The latter may occur when a patient is colonized with KPC at undetectable levels on admission and these bacteria grow to detectable levels during the patient’s LTACH stay. The relevance of this mechanism has also been demonstrated for extended-spectrum β- lactamase–producing bacteria.23 Different parameter estimates for the 4 LTACHs may have


arisen from inherent differences between them, eg, differences in size and referral patterns. The cohorting strategy should not have influenced the transmission parameters. Cohorting redu- ces the number of contacts between colonized and uncolonized patients, possibly mediated by healthcare workers, but the transmission probability given a contact between a colonized and an uncolonized patient is not mediated by cohorting. Only if adherence to the infection prevention bundle was higher in cohort wards than in non-cohort wards would cohorting have had a direct influence on transmission parameters. Therefore, we considered the number of acquisitions per 1,000 patient days to judge the cohorting effect. LTACHs B and D had the lowest numbers of acquisitions per 1,000 patient days, suggesting that their strategies were superior to the strategy of mixed-cohort


floors, as adopted at LTACHs A and C. However, 95%credible intervals of parameters for the 4 LTACHs overlapped, pre- cluding firmconclusions. The regression analysis indicated that the higher the adherence to cohorting, the lower the number of acquisitions. This result is another indication that separating KPC-positive from KPC-negative patients is a good control strategy for containing the spread of KPC.8,34 However, con- founding factors such as differences between LTACHs in hand hygiene compliance or case mix may still have played a role. A similar result was found by Ben-David et al.35 In their univariate analysis, lower adherence to placement of colonized patients in single rooms or cohorting was a risk factor for newly discovered CPE carriage in a long-term care facility. Creating a cohort for KPC control is intuitively attractive but can be difficult in practice. Our data provide an important demon- stration of the potential benefit that can be weighted against the effort needed to maintain the cohort. Although this benefit might be explained by physical separation of positive and negative patients, it may also be related to nurse cohorting, optimal bathing technique, andmore attention to hand hygiene. The finding that the estimates for swab sensitivity for all


4 LTACHs were similar and that all 95% credible intervals overlapped may reflect the fact that all swabs were analyzed in a central laboratory. There do not seem to be major differences among LTACHs in swabbing techniques used. There were differences between the number of acquisitions predicted by the model and the crude data, as observed in the epidemiological study.24 These differences can be partially explained by the sensitivity of KPC screening that is taken into account in theMCMCmodel. Because specificity was assumed to be 100%, model estimates for the number of acquisitions should be equal to or higher than study counts. Furthermore, in the MCMC model, cultures from previous admissions were ignored and patients who were considered KPC-positive on admission in the epidemiological study might have acquired KPC carriage in the model. Because patients may lose coloni- zation between discharge and readmission, assuming patients are still positive on admission leads to overestimation of the prevalence of colonization on admission. Because no infor- mation was available on colonization status during previous LTACH visits before the start of the study, we had incomplete data on previous colonization status. Therefore, our main model, which allows for colonization on admission, is most likely more accurate. Finally, no samples were obtained from some patients. These missing data were taken into account in the MCMC model. The sensitivity analyses demonstrated the robustness of the model estimates. Although we are not aware of similar studies in LTACHs,


Worby et al29 used a similar method to estimate transmission parameters of MRSA in 10 general hospital wards in the Uni- ted Kingdom. Naturally, differences in setting and pathogen characteristics preclude direct comparisons of study results. However, estimates for α and β, as obtained for KPC in LTACHs, were at least 10 times higher than those of MRSA in general hospital wards. This result might reflect the fact that


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