Progression From New MRSA Colonisation to Infection
National University Hospital (NUH) is a 1000 bed tertiary hospital in Singapore. An MRSA control bundle which included active surveillance cultures (ASC) at admission and discharge was introduced in Intensive Care Units (ICUs) in 2007. The bundle was progressively expanded throughout the hospital over subsequent years such that by 2011, universal ASC on ward admission and discharge were routine on all adult inpatient wards except psychiatry and obstetrics. Patients transferring between wards underwent ASC upon transfer (e.g. from ICU to general ward). Patients colonised with MRSA were not routinely decolonised during this study.
We conducted a retrospective cohort study to evaluate the risk of progression to infection in adult patients who had newly acquired MRSA during an inpatient admission to NUH between 1 July 2007 and 30 June 2011. Patients were included if they had negative ASC on admission but subsequent positive ASC during the same admission. Patients were excluded if they had been admitted to any healthcare facility within the preceding 12 months or if they had a history of previous MRSA colonisation or infection. Analysis was not performed on patients that developed MRSA infection during their index admission if the clinical isolate grew MRSA prior to the screen swab. That is, patients who appeared to develop infection without evidence of prior colonisation were not included. Only Singapore citizens or permanent residents were included in the study to avoid likely loss to follow up. Patient records were reviewed until death or 30 June 2012 to determine if clinical MRSA infection developed. In this way, we aimed to investigate the risk of progression to infection in a group of patients in whom we could reasonably attribute MRSA acquisition to a specific admission at our hospital.
ASC were taken using one swab from both nares, and one from both axillae and groin on admission. Swabs were processed as a pooled sample and inoculated on chromogenic media (MRSA Select, Bio-Rad Laboratories, Marnes-la-Coquette, France) with aerobic incubation for up to 48 hours. Clinical specimens were taken at the discretion of the primary team according to routine care and processed using existing laboratory protocols based on the type of sample. Prior to 2010, MRSA identification was confirmed using Vitek2 instrument (bioMérieux, Marcy L'Etoile, France), and from 2010 onwards, by matrix assisted laser desorption ionisation-time of flight (MALDI-TOF, Bruker Daltoniks GmbH, Bremen, Germany).
Data were collected retrospectively on demographics (age, gender, ethnicity) and pre-existing co-morbidities (diabetes, active malignancy, non-cancer immunosuppression, chronic haemodialysis). Data on healthcare utilisation during the admission in which MRSA was acquired were also collected. This was limited to information stored electronically but included ICU admission, surgery within 30 days of MRSA acquisition, and intravascular catheter placement. Co-morbidities were classified using International Classification of Diseases, Ninth Revision codes from discharge summary records. Patients developing MRSA clinical infection were further evaluated as to the primary site of the infection according to NHSN definitions, need for admission and number of days between first positive MRSA ASC and first positive culture from a clinical site (i.e. not a surveillance culture). Outcome data on re-admission due to MRSA infection at any public hospital in Singapore and for mortality at one and six months were reviewed.
The study was approved by the Domain Specific Review Board for the National Healthcare Group institutions in Singapore (NHG DSRB 2012/00513).
Bivariate analysis was first performed to identify variables that showed significance at the 0.20 level with a binary outcome, with the exceptions of age, gender and ethnicity which were forced into the model to address the possibility of residual confounding. To verify an association between categorical variables, the χ-square test was employed. Akaike and Bayesian Information Criterion (AIC/BIC) were used as primary drivers for model building, and the model fit was checked with the Hosmer-Lemeshow goodness-of-fit test (by deciles). Additionally, receiver-operating curve (ROC) plots and sensitivity/specificity tables were utilised to assess the practical utility of the individual logistic models. Kaplan-Meier analysis was performed to identify the unadjusted time-to-event with associated confidence intervals within the entire cohort. Data analysis was performed using Stata 12.1 (College Station, Texas); all tests were two-tailed.
Methods
Setting
National University Hospital (NUH) is a 1000 bed tertiary hospital in Singapore. An MRSA control bundle which included active surveillance cultures (ASC) at admission and discharge was introduced in Intensive Care Units (ICUs) in 2007. The bundle was progressively expanded throughout the hospital over subsequent years such that by 2011, universal ASC on ward admission and discharge were routine on all adult inpatient wards except psychiatry and obstetrics. Patients transferring between wards underwent ASC upon transfer (e.g. from ICU to general ward). Patients colonised with MRSA were not routinely decolonised during this study.
Design
We conducted a retrospective cohort study to evaluate the risk of progression to infection in adult patients who had newly acquired MRSA during an inpatient admission to NUH between 1 July 2007 and 30 June 2011. Patients were included if they had negative ASC on admission but subsequent positive ASC during the same admission. Patients were excluded if they had been admitted to any healthcare facility within the preceding 12 months or if they had a history of previous MRSA colonisation or infection. Analysis was not performed on patients that developed MRSA infection during their index admission if the clinical isolate grew MRSA prior to the screen swab. That is, patients who appeared to develop infection without evidence of prior colonisation were not included. Only Singapore citizens or permanent residents were included in the study to avoid likely loss to follow up. Patient records were reviewed until death or 30 June 2012 to determine if clinical MRSA infection developed. In this way, we aimed to investigate the risk of progression to infection in a group of patients in whom we could reasonably attribute MRSA acquisition to a specific admission at our hospital.
ASC were taken using one swab from both nares, and one from both axillae and groin on admission. Swabs were processed as a pooled sample and inoculated on chromogenic media (MRSA Select, Bio-Rad Laboratories, Marnes-la-Coquette, France) with aerobic incubation for up to 48 hours. Clinical specimens were taken at the discretion of the primary team according to routine care and processed using existing laboratory protocols based on the type of sample. Prior to 2010, MRSA identification was confirmed using Vitek2 instrument (bioMérieux, Marcy L'Etoile, France), and from 2010 onwards, by matrix assisted laser desorption ionisation-time of flight (MALDI-TOF, Bruker Daltoniks GmbH, Bremen, Germany).
Data were collected retrospectively on demographics (age, gender, ethnicity) and pre-existing co-morbidities (diabetes, active malignancy, non-cancer immunosuppression, chronic haemodialysis). Data on healthcare utilisation during the admission in which MRSA was acquired were also collected. This was limited to information stored electronically but included ICU admission, surgery within 30 days of MRSA acquisition, and intravascular catheter placement. Co-morbidities were classified using International Classification of Diseases, Ninth Revision codes from discharge summary records. Patients developing MRSA clinical infection were further evaluated as to the primary site of the infection according to NHSN definitions, need for admission and number of days between first positive MRSA ASC and first positive culture from a clinical site (i.e. not a surveillance culture). Outcome data on re-admission due to MRSA infection at any public hospital in Singapore and for mortality at one and six months were reviewed.
The study was approved by the Domain Specific Review Board for the National Healthcare Group institutions in Singapore (NHG DSRB 2012/00513).
Statistical Analysis
Bivariate analysis was first performed to identify variables that showed significance at the 0.20 level with a binary outcome, with the exceptions of age, gender and ethnicity which were forced into the model to address the possibility of residual confounding. To verify an association between categorical variables, the χ-square test was employed. Akaike and Bayesian Information Criterion (AIC/BIC) were used as primary drivers for model building, and the model fit was checked with the Hosmer-Lemeshow goodness-of-fit test (by deciles). Additionally, receiver-operating curve (ROC) plots and sensitivity/specificity tables were utilised to assess the practical utility of the individual logistic models. Kaplan-Meier analysis was performed to identify the unadjusted time-to-event with associated confidence intervals within the entire cohort. Data analysis was performed using Stata 12.1 (College Station, Texas); all tests were two-tailed.
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