Impact of comorbidity on risk of venous thromboembolism in patients with breast cancer: a Danish population-based cohort study

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.