Occurrence of seasonal variation in incident unprovoked venous thromboembolism in the Danish population

Introduction: Incidence of venous thromboembolism (VTE), i.e. deep venous thrombosis and pulmonary embolism, is found to be higher in winter than summer. However, this variation may result from seasonal variation in occurrence of diseases that provoke VTE. Knowledge about possible seasonal variation of incident VTE may improve preventive strategies. 46

Objectives: We aimed to asses the occurrence of seasonal variation in incident unprovoked VTE using the Danish population as a cohort.

Materials and methods: A cohort consisting of the Danish population aged 20 or more from 1980-2008. We excluded subjects with a previous diagnosis of cancer or with any discharge diagnosis until three months before an incident VTE diagnosis. Daily data including information on gender, age and comorbid diseases are analysed. A Poisson regression model adjusting for gender, age- and Charlson comorbidity index groups is fitted to data. Seasonal variation is modelled with a single cycle sinusoidal curve interacting with comorbidity groups. Secular trend is modelled with a restricted cubic spline. The relative risk is estimated by the peak-to-trough ratio with bootstrap bias-correcting 95% confidence intervals.

Results: We identified 101,823 subjects having an incident discharge diagnosis of VTE using the Danish National Patient Registry. The estimated relative risk of incident rate of VTE for subjects without comorbid diseases was 1.18 (95% CI: 1.16-1.21), peaking in January, and 1.02 (95% CI: 1.00-1.03) for both remaining groups. Stratification on gender showed no significant difference in relative risk for any comorbidity groups. For subjects without comorbid diseases the relative risk tended to increase with age.