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heterogeneity in risk of poor outcomes exists because it may impact how CDI prevention efforts should be targeted.
Methods
We obtained data for CDI patients from 2010–2012 Medicare claims data for all persons coded for CDI in 2011. For control patients, we used the 2010–2012 5% random sample Medicare data, excluding persons coded for CDI in 2011.16 Long-term care facility (LTCF) stays were identified using the 2010–2012 mini- mum data set (MDS), which includes standardized assessments of patients in nursing facilities that accept federal payment.17 Eligible patients were those aged ≥66 years with complete
Medicare fee-for-service enrollment during the 12 months prior to the CDI (and control) index date. Persons with no claims in 2010 and 2011 were excluded to ensure use of health benefits. Patients coded for CDI in the last quarter of 2010 were excluded to restrict the population to individuals newly diagnosed in 2011.
Index date
Patients coded for CDI (International Classification of Disease, Ninth Revision, Clinical Modification [ICD-9-CM] diagnosis code 008.45) were identified from January 1, 2011, through December 31, 2011, in the inpatient, outpatient, carrier (ie, physician), or skilled nursing facility (SNF) files. The CDI onset date was assigned as described previously,18 and an analogous index date was randomly selected for control patients such that the dis- tribution of index dates among control patients mirrored the distribution of CDI index dates.19 The first episode of CDI in 2011 was used for all analyses.
Outcomes
All-cause mortality within 1 year was identified using the death date in the Beneficiary Summary file. Secondary outcomes included new transfer to an LTCF, new transfer to an SNF, and 1 or more hospitalizations within 30 days, 90 days, and 1 year after the index date. New transfer to a SNF was identified using the Medicare SNF file. LTCF residence was identified using method 2 described by Goodwin et al,20 that is, using the SNF file and MDS to distinguish long-term from short-term SNF encounters.17,20 For new transfers to SNF and LTCF, patients were excluded if they met the definition for these encounters in the year prior to the index date. Acute-care hospitalizations with admission date after the index
date were identified using the inpatient file. For individuals hospitalized on their index date, same-day transfers to another hospital were excluded because they were not new hospitaliza- tions. A subsequent hospitalization to treat CDI in patients diagnosed as an outpatient was considered a new hospitalization.
Covariates
Risk factors for CDI in the year prior to the index date were identified using ICD-9-CM diagnosis, Current Procedural Ter- minology, 4th edition, and uniform billing revenue codes. Risk factors included comorbidities, infections, and healthcare expo- sures, as defined previously,19 and acute noninfectious conditions and frailty indicators (Appendix Table 1). Acute noninfectious conditions included conditions expected to require hospitaliza- tion or outpatient treatment that may result in antibiotic exposure
Margaret A. Olsen et al
(Appendix Table 1). Frailty indicators were conditions associated with declining health (eg, decubitus ulcer, difficulty walking) (Appendix Table 1). Comorbidities were identified as recom- mended by Klabunde et al,21 whereas acute conditions required only a single code.
Statistical analysis
Descriptive statistics were performed using the χ2 and Mann– Whitney U tests. To create the propensity score, we used multi- variable logistic regression with the dependent variable CDI and independent variables in Appendix Table 2. To calculate the probability of CDI at the index date, the independent variables were assessed in the year prior. The logit of the propensity score was used to match cases and controls 1:1 without replacement, using a caliper of 0.2 times the standard deviation of the logit of the propensity score.22,23 Standardized differences for all covari- ates were calculated before and after matching, with values >0.1 indicating imbalance (Appendix Fig. 1).24,25 We used the McNemar test to compare mortality, LTCF, and
SNF transfer, with calculation of odds ratios for matched pairs. For the LTCF analysis the population was restricted prior to matching to exclude individuals previously residing in a LTCF and individuals hospitalized at their index date who died during the hospitalization, because they would not have the opportunity for an LTCF transfer. The population was restricted similarly for the SNF analysis. Attributable risk was calculated in the matched pairs by subtraction of the percentage of controls with outcome from the percentage of CDI cases with outcome. For analysis of subsequent hospitalizations, Cox proportional hazards models were performed with a robust variance estimator to account for the matching.26 For stratified analyses, the probabilities were divided into 20
strata (ie, ventiles), based on the propensity score in the CDI group to obtain approximately equal numbers of CDI cases across strata for analysis of mortality, which resulted in variable num- bers of control patients per stratum. For secondary analyses, because individuals were excluded based on specific criteria, the numbers of CDI cases were no longer equal across strata. Ana- lyses were performed using Cox proportional hazards models, with CDI the only independent variable, stratified by the pro- pensity score ventiles. SAS version 9.4 software (SAS Institute, Cary, NC) was used for all analyses. The Washington University Human Research Protection Office approved this research with a waiver of informed consent.
Results
The population of fee-for-service Medicare beneficiaries aged 66 and older included 1,510,046 persons. Of these, 16,605 were excluded due to CDI diagnosed in the last quarter of 2010, resulting in a final population of 1,493,441 persons for analysis: 174,903 coded for CDI and 1,318,538 control patients. Extra- polation to the entire 2011 fee-for-service Medicare population with at least 1 healthcare claim in 2010–2011 resulted in a comparison population of ~26.4 million and an estimated CDI incidence of 663 per 100,000 elderly persons. The mean age of the study population was 77.5 years; 925,316
patients (62.0%) were women; and 1,301,397 patients (87.1%) were white (Table 1). Also, 271,128 patients (18.2%) had dual eligibility in Medicare and Medicaid, indicative of low
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