Income-related differences in mortality among people with diabetes mellitus

Background

Mortality has declined substantially among people with diabetes mellitus over the last decade. Whether all income groups have benefited equally, however, is unclear. We examined the impact of income on mortality trends among people with diabetes.

Methods

In this population-based, retrospective cohort study, we compared changes in mortality from Apr. 1, 1994, to Mar. 31, 2005, by neighbourhood income strata among people with diabetes aged 30 years or more in the province of Ontario, Canada.

Results

Overall, the annual age- and sex-adjusted mortality declined, from 4.05% in 1994/95 (95% confidence interval [CI] 3.98%–4.11%) to 2.69% in 2005/06 (95% CI 2.66%–2.73%). The decrease was significantly greater in the highest income group (by 36%) than in the lowest income group (by 31%; p < 0.001). This trend was most pronounced in the younger group (age 30–64 years): the mortality rate ratio widened by more than 40% between the lowest and highest income groups, from 1.12 to 1.59 among women and from 1.14 to 1.60 among men. Income had a much smaller effect on mortality trends in the older group, whose drug costs are subsidized: the income-related difference rose by only 0.9% over the study period.

Interpretation

Mortality declined overall among people with diabetes from 1994 to 2005; however, the decrease was substantially greater in the highest income group than in the lowest, particularly among those aged 30–64 years. These findings illustrate the increasing impact of income on the health of people with diabetes even in a publicly funded health care setting. Further studies are needed to explore factors responsible for these income-related differences in mortality.The number of people with diabetes mellitus has increased dramatically over the last 20 years^1,2 and is estimated to double to about 366 million by 2030.³ Diabetes is associated with a 2-fold increase in mortality, with the majority of deaths attributed to cardiovascular causes.⁴ However, survival among people with diabetes has improved substantially over the last decade,^1,5,6 in part because of better diabetes care and a reduction in cardiovascular events.⁶Income is a well-known predictor of survival.^7,8 Even in Canada, where much of health care is universally funded, income-based inequities in health and access to care remain.^9–12 Although income-related differences in all-cause mortality have decreased since the advent of provincially subsidized health care in Canada,^8,9 the income gap may be increasing for certain causes of death, including those related to diabetes.⁸ The shift to more complex medical care involving a greater number of drug therapies has resulted in improved diabetes-related outcomes overall.¹³ However, patients in lower-income groups may not have benefited from advances in diabetes care as much as more affluent patients have because of the financial burden of out-of-pocket expenses for such medications and diabetes supplies.We conducted a population-based study to examine income-related differences in mortality from 1994 to 2005 among people with diabetes.