Socioeconomic status (SES) and 30-day hospital readmissions for chronic obstructive pulmonary (COPD) disease: A population-based cohort study

Background Patients with chronic obstructive pulmonary disease (COPD) are more likely to be readmitted than patients with other chronic medical conditions, yet knowledge regarding such readmissions is limited. We aimed to determine factors associated with readmission within 30 days of a COPD hospitalization or death with an emphasis on examining aspects of socioeconomic status and specific comorbidities. Methods A population-based cohort study was conducted using health administrative data from Ontario, Canada. All hospitalizations for COPD between 2004 and 2014 were considered. The primary exposures were socioeconomic status as measured by residential instability (an ecologic variable), and comorbidities such as cardiovascular disease and cancer. Other domains of socioeconomic status were considered as secondary exposures. Logistic regression with generalized estimating equations was used to examine the effect of exposures, adjusting for other patient factors, on 30-day readmission or death. Results There were 126,013 patients contributing to 252,756 index COPD hospitalizations from 168 Ontario hospitals. Of these hospitalizations, 19.4% resulted in a readmission and 2.8% resulted in death within 30 days. After adjusting for other factors, readmissions or death were modestly more likely among people with the highest residential instability compared to the lowest (OR 1.05, 95% CI 1.01–1.09). Comorbidities such as cardiovascular disease and cancer, as well as other aspects of low socioeconomic status also increased readmission or death risk. Interpretation Socioeconomic status, measured in various ways, and many comorbidities predict 30-day readmission or death in patients hospitalized for COPD. Strategies that address these factors may help reduce readmissions and death.

discharge disposition; where length of stay was an outlier (greater than the 99th percentile); where patients were transferred to a second acute care hospital on discharge; or where patients died were excluded. Individuals could have more than one index hospitalization. Hospitalizations that occurred within 30 days of a previous hospitalization were considered readmissions and not index hospitalizations. The exception to this was in the calculation of repeated readmissions.

Readmissions
Readmissions with pregnancy, childbirth, or puerperium as the most responsible diagnosis were not considered. If there were multiple readmissions within the 30-day period, only the first was used to calculate and predict readmission rates. All, however, were counted to examine rates of multiple readmissions within 30 days. COPD-specific readmissions were considered in secondary analyses

Exposure Outcomes
SES was characterized using the four domains of the validated Ontario Marginalization Index that capture processes by which individuals and groups are prevented from fully participating in society. 20 Residential instability considers the proportion of people living in an area who are alone or youth, as well as number of persons per dwelling, number of people living in apartments, and number of people who are married. Material deprivation, another domain, considers the proportion of people with no high school graduation, who are from lone parent families, who are unemployed, and/or who are low income. Ethnic concentration considers the proportion of people who are recent immigrants and visible minorities. Finally, Dependency considers the ratio of seniors and children to working age adults and labor force participation.

Additional Analyses
To determine if results were consistent when the competing risk of death was not considered, we repeated the main analysis with the outcome of 30-day all-cause readmissions alone. To determine if COPD specific readmission differed from readmissions due to any cause, we performed additional analyses substituting the former for the later. Secondary analyses were completed examining readmission rates or death within 90 days and stratifying the primary analysis by younger (under 65 years) and older age.

Additional Analyses
Risk factors for all-cause readmissions (but not death) and their magnitudes (including the four domains of SES marginalization and income quintile) were similar to the risk factors for allcause readmissions and death except dementia was no longer a significant risk factor ( Table 3).
Risk factors for COPD-specific readmissions and their magnitudes (including the four domains of SES marginalization and income quintile) were largely similar to all-cause readmissions and death, except for comorbidities where only congestive heart failure, asthma and cancer were significantly associated with increased COPD readmissions. People age 65 and older seemed less affected by SES factors than those under 65 (e-

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