Hospitalizations for chronic obstructive pulmonary disease (COPD) are widely considered to be potentially avoidable through effective management in primary care [1–3]. Differences across population groups in the risk of admission (and readmission) for COPD and other ambulatory care sensitive conditions (ACSCs) provide an indirect measure of the capacity of the healthcare system to manage chronic conditions, and help outline inequalities related to access for primary care services for early diagnosis and prevention of acute exacerbation [3, 4]. There is strong evidence to support the notion that large income differences at the societal level have negative effects on health, and specifically that socioeconomic status is inversely associated with COPD outcomes even in high-income countries [5, 6]. In the Canadian context, although primary care physician services are universally accessible based on medical need rather than ability to pay through the stipulation of the Canada Health Act, and thus should be resistant to income inequalities, the equality of accessing primary care services may be amenable to improvements if there exists differential levels of access based on income. The effects of these unequal health outcomes for persons with COPD are experienced as reduced levels of breathing capabilities, decreased ability for the individual to leave home, and increased difficulty with activities of daily living, all of which may contribute to acute needs requiring hospitalizations if primary care services are not available, accessible and acceptable [2, 7].

Prevalence estimates of COPD in Canada vary depending on the information source. Survey data based on self-reports of physician-diagnosed COPD suggest a prevalence rate of 4.1% (95% confidence interval [CI]: 3.8–4.3%) among Canadian adults in 2018 [8]. However, comparisons from those who had a measured airflow obstruction consistent with COPD indicate the disease is under-reported and under-diagnosed, with a “true” prevalence closer to 14.1% (95% CI: 11.6–17.0%) among those aged 35-79 years [9]. A study using linked survey and hospitalization data found that Canadians aged 12-74 admitted for any one of seven ACSCs represent 0.4% of the population, with COPD accounting for one-quarter (26%) of such potentially avoidable hospitalizations [3]. Increases in the burden of COPD over time have been attributed to a lack of improvement in smoking rates and increased hospitalization rates in the lowest income group [10]. Studies have also pointed to persistent and increasing income-related disparities in COPD mortality [11].

Understanding inequities in health outcomes for persons with COPD can help inform the development of medical and non-medical interventions for this at-risk population. Hospitalizations for COPD are commonly hypothesized to serve as a proxy for inadequate primary care access, quality and coordination, notably given that effective pharmacological treatment can be delivered in primary care, which should lower the risk of acute-care admissions [12, 13]. Having follow-up in primary and community-based care after discharge is presumed to reduce readmission risks; however, there is little robust empirical evidence explaining why hospitalization and rehospitalization rates vary across population groups [14]. Patient sociodemographic characteristics have been found to play a role in COPD admissions and readmissions in Canada, although the literature is inconclusive, with variances in observed outcomes reflecting differences across studies in target population, methodological approach and clinical need [2, 14]. Some evidence has been found of an association between low income and ACSC-related hospitalization even under universal health coverage, largely derived from either ecological studies or from analyses grouping multiple categories of illness [3].

The ability to investigate which population subgroups are most at risk of an ACSC-related admission to inform prevention interventions has been enhanced in recent years through the increased availability of linked datasets from nationally representative household-based sources with hospital discharge records [3, 15]. Studies relying on social surveys may be able to consider a range of variables such as health status and health-related behaviours, but sample size limitations often preclude assessment of cause-specific hospital stays [15]. The use of linked data from the Canadian Population Census offers the benefit of a large sample size to provide more detailed investigation of the influences of the social determinants of health (e.g. income) on potentially avoidable COPD hospitalizations and rehospitalizations. The influence of patients’ social context on COPD readmissions has been rarely examined [16]. The objective of this research is to examine the extent to which income influences the risk of acute-care hospitalizations and rehospitalizations for COPD, after controlling for other sociodemographic characteristics, leveraging data from the 2006 Census long-form questionnaire linked longitudinally to three years of hospital administrative records.

The study cohort included 2.41 million individuals aged 30-69 who completed the long-form census and were eligible to be matched to the DAD. This sample was designed to represent 11.9 million Canadians of the same age range residing in one of nine provinces (excluding Quebec). Just over half (51.1%) of the target population were female, and 42.8% were between the ages of 50 and 69 at the baseline time of the census (Figure 1). By definition, 20% of the cohort fell in each of the five income quintiles. One in fifteen (6.4%) were of Aboriginal identity, and 27.9% were foreign-born. Individuals with at most a high school diploma represented 42.0% of the population, and 66.3% were married or in a common-law union.

Among the target population, 0.22% were admitted to hospital at least once for COPD during the study period. Of these, 0.39% experienced a COPD-related readmission within six months.

Results of the multiple logistic regressions for the risk of COPD hospitalization and COPD readmission within six-months are presented in Table 1. A direct and significant income gradient for risk of ever-hospitalized for COPD was found (Table 1, model 1). Compared with those in the highest income quintile, adults in the lowest income quintile had 2.92 times the odds (95% CI: 2.77–3.10) of experiencing a COPD admission, those in the second quintile had 2.01 times the odds (95% CI: 1.90–2.13), the odds ratio for those in third quintile was 1.74 (95% CI: 1.64–1.85), and the OR for those in the fourth income quintile was 1.38 (95% CI: 1.30-1.47).

As expected, the risk of admission for COPD increased significantly with age; the odds increased by 12% with each additional year of age. Being female was somewhat protective of the hospitalization risk (3% less odds compared to males), as was being married or in a union (the odds were approximately 34%–64% higher among those not in union). The admission risk decreased significantly with level of education; compared with those who did not complete high school, individuals who had a high school diploma had 41% decreased odds of being hospitalized for COPD, those who had postsecondary diploma had 44% decreased odds, and those who had a university degree had 76% decreased odds. Individuals of Aboriginal identity had 56% greater odds of hospitalization due to COPD compared with non-Aboriginals, after controlling for income, education and other socio-demographics. Those who were foreign-born had 63% decreased odds of a COPD hospitalization compared with non-immigrants. Compared with residents of Ontario (the most populous province), individuals from Newfoundland and Labrador, Nova Scotia, Manitoba, and British Columbia had lower odds of hospitalization for COPD, while those from New Brunswick or Prince Edward Island (two of the least populous and most rural provinces) had significantly higher odds, and those from Saskatchewan and Alberta had statistically similar odds.

The second analysis showed that individuals’ income group was a significant predictor of the risk for six-month COPD readmission (Table 1, model 2), albeit the gradient was less pronounced compared to the risk for ever-hospitalization. Compared with individuals in the wealthiest income quintile, those in the least affluent group had 1.39 (95% CI: 1.22–1.58) times the odds of readmission, those in the third income quintile had 1.28 (95% CI: 1.11–1.47) times the odds, and the OR for those in the fourth income quintile was 1.17 (95% CI: 1.00–1.36).

A person’s age remained a significant predictor of COPD rehospitalization (OR: 1.03; 95% CI: 1.02–1.03), but their sex did not significantly influence the risk. Aboriginal identity was not a significant predictor for risk of rehospitalization. Compared with those who were married or in union, individuals who were formerly or never married had approximately 18%–25% greater odds of readmission. Level of education attained showed a social gradient in the risk of rehospitalization: compared with those who did not complete high school, the odds decreased among individuals who had a high school diploma (OR: 0.88; 95% CI: 0.82–0.94), those with post-secondary diploma (OR: 0.83; 95% CI: 0.78-0.89), and those with a university degree (OR; 0.85; 95% CI: 0.76–0.96). Immigrants had reduced odds of readmission (OR: 0.87; 95% CI: 0.79–0.94) compared with non-immigrants. Compared with residents of Ontario, individuals who lived in Nova Scotia, New Brunswick or Prince Edward Island, and Alberta had decreased odds of rehospitalization, while those in British Columbia had higher odds and those in Newfoundland and Labrador, Manitoba and Saskatchewan had statistically similar odds.

This study utilized multiple years of hospital inpatient data linked longitudinally with population census data to investigate the associations between income inequalities and risks of COPD-related admissions and readmissions among Canadian adults. Health inequalities by income have historically been examined in Canada using area-based approaches; the use of census linked datasets offers a more robust tool to advance understanding of gradients in health for specific conditions and outcomes [21]. The present results demonstrated a significant social gradient for household income on COPD admission risks. After controlling for age, sex, education and other socio-demographics, an independent association remained between the risk for potentially avoidable COPD hospitalization and relative household income. Compared with individuals in the most affluent income group, those in the lowest income quintile were nearly three times as likely to be hospitalized (OR: 2.92; 95% CI: 2.77–3.10), and those in the second lowest quintile were twice as likely (OR: 2.01; 95% CI: 1.90–2.13). Similarly, income was found to be an important predictor of the risk for six-month COPD readmission, albeit with a less pronounced gradient: compared with the wealthiest group, the odds of readmission remained significantly higher among those in the lowest income quintile (OR: 1.39; 95%CI: 1.22–1.58) and the second income quintile (OR: 1.15; 95%CI: 1.00–1.31).

These results add to the growing body of literature demonstrating significant associations between income inequalities and the risk of potentially avoidable hospitalization for chronic non-communicable diseases, even in contexts of publicly-funded universal healthcare coverage. Previous Canadian studies using linked datasets have found that hospitalizations for diabetes and other ACSCs were more likely to be in the lowest household income quintile [3, 10, 15]. In England, socioeconomic deprivation was demonstrated to be one of the variables with the highest explanatory power in predicting COPD hospitalizations [22]. In Italy, an assessment of the relationship of income with the risk of ACSC-related hospitalizations found the strongest income gradient for COPD admissions, with a relative risk of 4.23 in the lowest income quintile compared with the highest quintile [23]. Such findings may reflect disparities in individuals’ health literacy, lived experiences and ability to navigate the healthcare system, undiagnosed conditions, smoking cessation, among other unmeasured factors [22, 23].

Consistent with research elsewhere, the present findings found a protective influence of having a marital companion on odds of COPD hospitalization and readmission, which may reflect social capital within a household and informal caregiving support for individuals to self-manage their disease, making them less susceptible to acute exacerbations [3, 16]. Our study demonstrated an inverse relationship between level of education and risk for COPD hospitalization – similar to evidence from the United States, where the hazard ratio was found to be 1.5 times higher for the lowest education group [24]. We also observed a protective role of immigrant status on adverse chronic disease outcomes, consistent with the literature on the “healthy immigrant effect” [25]. On the other hand, we found a person’s sex to be only somewhat associated with differences in COPD hospitalizations; international data from OECD countries pointed to men having a 53% higher hospitalization rate for COPD than women [12].

We did not control for tobacco use in our analyses, although smoking status is often included in both population- and clinical-based studies of COPD outcomes. This was related to the lack of information on behavioural risk factors in the census as well as the likely multi-collinearity between income and tobacco use. Moreover, research elsewhere has pointed to unexpected observations of the associations between smoking and acute cardiovascular-related outcomes (and other high-mortality problems), attributable to selective survival bias from the (unmeasured) earlier net probability of death from tobacco use before hospitalization in health assessments using population-based data [26, 27].

This study presented the first comprehensive national investigation into the associations between income inequalities and the cause-specific risk of hospitalization and the risk of six-month readmission for COPD among a population-representative cohort of Canadian adults. Due to the significant influence of income inequality on likelihood of potentially avoidable hospitalizations for COPD found in this study, it can be inferred that universal public health insurance in Canada continues to inadequately address the social determinants of health, widely postulated as crucial for reducing health inequalities and persistent unmet medical needs of the most vulnerable populations [31]. The exclusion of the long form in the 2011 quinquennial Canadian census [32], coupled with the widely acknowledged time lags between the collection and linkage of data sources and their readiness of use for research purposes [30], meant that only data from the 2006 census linked to the DAD were available at the time of this study. Given the emerging conflicting evidence regarding improvement in ambulatory care to prevent ACSC-related hospitalizations, future research should seek to better understand changes over time in hospital-based COPD outcomes and whether variations in the use of primary care and preventive services within and across income groups are able to explain the differences, and if so, which policy actions at the federal and provincial levels are most amenable to advancing health equity for all.

This study complied with the University of New Brunswick’s Research Ethics Board, which does not require an additional institutional review for research projects using data accessed through the NB-RDC.