Summary

Deep vein thrombosis (DVT) is a common disease with anannual incidence of about 1 in 1000. Many risk factors havealready been studied, both genetic and acquired. It is unclearwhether obesity affects thrombotic risk in unselected patients.Obesity is common, with a prevalence of 20-25% and maytherefore have a considerable impact on the overall incidenceof thrombosis.We evaluated the risk of thrombosis due tooverweight and obesity using data from a large populationbased case-control study.Four hundred and fifty-four consecutive patients with a firstepisode of objectively diagnosed thrombosis from threeAnticoagulation Clinics in the Netherlands were enrolled in acase-control study. Controls were matched on age and sex topatients and were introduced by the patients.All patients completeda standard questionnaire and interview, with weight andheight measured under standard conditions.The associations ofobesity with clotting factor levels were studied to investigatepossible mechanisms.Obesity (BMI ≥30 kg/m²) increased the risk of thrombosistwofold (CI95: 1.5 to 3.4), adjusted for age and sex. Obese individualshad higher levels of factor VIII and factor IX, but not offibrinogen.The effect on risk of obesity was not changed afteradjustment for coagulation factors levels (fibrinogen, F VIII, F IX,D-dimer). The relative risk estimates were similar in differentage groups and in both sexes, indicating a larger absolute effectin older age groups. Evaluation of the combined effect of obesityand oral contraceptive pills among women aged 15-45revealed that oral contraceptives further increased the effect ofobesity on the risk of thrombosis, leading to 10-fold increasedrisk amongst women with a BMI greater than 25 kg/m² whoused oral contraceptives.Obesity is a risk factor for deep vein thrombosis. Amongwomen with a BMI greater than 25 kg/m² the synergistic effectwith oral contraceptives should be considered when prescribingthese.