Additive interaction of disability with chronic conditions on mortality risk in middle-aged and older adults in Spain.

Background: Disability and chronic diseases are prevalent conditions associated with mortality, but little information is available on their potential synergistic effects.

Objective: This study aimed to describe additive interactions between disability and chronic diseases on mortality risk in middle-aged and older adults.

Methods: A representative cohort of 22,800 community-dwelling Spanish people aged 50 years or older were interviewed for disability with the Global Activity Limitation Indicator and specific chronic diseases in the 2011-12 and 2014 National Health Surveys and subsequently followed up for mortality. Five-year all-cause mortality risks were standardized in each disability-by-comorbidity category through inverse probability weighting. We computed interaction contrasts as the departure of the standardized risk difference for people with both conditions from the sum of the standardized risk differences for those with any single condition.

Results: The baseline prevalence of disability was 35.1 % (95 % confidence interval [CI] 34.4 %, 35.9 %). There was compelling evidence of synergistic effects of disability with chronic liver disease, heart diseases other than myocardial infarction, cancer, and cerebrovascular disease, with large positive interaction contrasts (95 % CIs) of 106.7 (-16.4, 229.9), 45.7 (6.9, 84.5), 45.1 (-15.0, 105.2), and 42.9 (-41.0, 126.9) excess deaths per 1000 persons. Less clear synergistic responses were observed for other comorbidities. We found some evidence of antagonism for osteoporosis, with a negative interaction contrast of -18.0 (95 % CI -82.2, 46.2) deaths per 1000 persons.

Conclusion: Given the high mortality risk in people with disability, the study of its synergistic effects with target comorbidities can provide relevant information regarding preventive measures.