Impact of comorbidity on the association between socioeconomic factors and long-term mortality of intensive care patients

Purpose We aimed to estimate the association between socioeconomic status and long-term mortality after intensive care, with a particular focus on the impact of comorbidity. Methods First admissions to the ICU, registered in the national Swedish intensive care register between 2009 to 2012, were linked to information on education and income. We estimated the association between these socioeconomic factors and mortality, using Cox regression with follow-up until 2016. Comorbidity was measured with a multi-dimensional comorbidity measure based on hospital discharge diagnoses. Results We identified 101 745 ICU patients ≥30 years old. The group with only elementary school had a higher mortality rate than the group with the highest educational level (adjusted HR, 1.20; 95% CI 1.16-1.23). The association was not notably influenced by adjustment for comorbidity. In a landmark analysis, the association was weaker during the first year after ICU admission (adjusted HR, 1.14; 95% CI 1.09-1.18) than after the first year (adjusted HR, 1.28; 95% CI 1.23-1.35). The associations were stronger in patients with lower comorbidity burden than in patients with more severe comorbidity. The associations were largely consistent when income was used to indicate socioeconomic status. Conclusion Low educational level was associated with an increased long-term mortality rate after ICU admission, but the association was not notably related to comorbidity. The association was stronger after the first year of follow-up, suggesting that it may be more related to unmeasured background characteristics such as lifestyle factors that are not reflected in measurable comorbidity rather than the ICU admission.