Background:Knowledge about prevalent and deadly combinations of multimorbidity is
needed.Objective:To determine the nationwide prevalence of multimorbidity and estimate
mortality for the most prevalent combinations of one to five diagnosis
groups. Furthermore, to assess the excess mortality of the combination of
two groups compared to the product of mortality associated with the single
groups.Design:A prospective cohort study using Danish registries and including 3.986.209
people aged ≥18 years on 1 January, 2000. Multimorbidity was defined as
having diagnoses from at least 2 of 10 diagnosis groups: lung,
musculoskeletal, endocrine, mental, cancer, neurological, gastrointestinal,
cardiovascular, kidney, and sensory organs. Logistic regression (odds
ratios, ORs) and ratio of ORs (ROR) were used to study mortality and excess
mortality.Results:Prevalence of multimorbidity was 7.1% in the Danish population. The most
prevalent combination was the musculoskeletal–cardiovascular (0.4%), which
had double the mortality (OR, 2.03) compared to persons not belonging to any
of the diagnosis groups but showed no excess mortality (ROR, 0.97). The
neurological–cancer combination had the highest mortality (OR, 6.35), was
less prevalent (0.07%), and had no excess mortality (ROR, 0.94).
Cardiovascular–lung was moderately prevalent (0.2%), had high mortality (OR,
5.75), and had excess mortality (ROR, 1.18). Endocrine–kidney had high
excess mortality (ROR, 1.81) and cancer–mental had low excess mortality
(ROR, 0.66). Mortality increased with the number of groups.Conclusions:All combinations had increased mortality risk with some of them having up to
a six-fold increased risk. Mortality increased with the number of diagnosis
groups. Most combinations did not increase mortality above that expected,
that is, were additive rather than synergistic.