Objectives: To assess the interaction between comorbidity and breast cancer (BC) on the rate of venous thromboembolism (VTE) beyond what can be explained by the independent effects of BC and comorbidity. Design: Population-based matched cohort study. Setting: Denmark. Participants: Danish patients with BC (n=62 376) diagnosed in 1995-2010 and a comparison cohort of women without BC (n=304 803) from the general population were matched to the patients with BC on year of birth in 5-year intervals and on the specific diseases included in the Charlson Comorbidity Index (CCI) and atrial fibrillation and obesity. Measures: The rate ratios of VTE per 1000 person-years (PY) were computed by comorbidity levels using the CCI, and interaction contrasts (IC) were calculated as a measure of the excess or deficit VTE rate not explained by the independent effects of BC and comorbidity. Results: Among patients with BC with a CCI score of 1, the 0-1 year VTE rate was 12/1000 PY, and interaction accounted for 10% of the rate (IC=3.2, 95% CI 0.5 to 5.9). Among patients with BC with CCI ≥4, the VTE rate was 17, and interaction accounted for 8% of the rate (IC=1.2, 95% CI -1.8 to 4.2). There was no interaction during 2- 5 years of follow-up. Conclusions: There was only little interaction between BC and the CCI score on the rate of VTE.
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