Hematocrit above the normal range for the population, such as in primary or secondary erythrocytosis, predisposes to both arterial and venous thrombosis. However, little is known about the association between hematocrit and risk of venous thromboembolism in a general population.

Hematocrit and related hematologic variables such as hemoglobin, red blood cell count, mean corpuscular volume, and baseline characteristics were measured in 26,108 subjects, who participated in the Tromsø Study in 1994-1995. Incident venous thromboembolic events during follow-up were registered up to September 1(st), 2007.

There were 447 venous thromboembolic events during a median of 12.5 years of follow-up. Multivariable hazard ratios per 5% increment of hematocrit for the total population, adjusted for age, body mass index and smoking, were 1.25 (95% CI: 1.08-1.44) for total venous thromboembolism and 1.37 (95% CI: 1.10-1.71) for unprovoked venous thromboembolism. In category-based analyses, men with a hematocrit in the upper 20(th) percentile (>or=46% in men) had a 1.5-fold increased risk of total venous thromboembolism (95% CI: 1.08-2.21) and a 2.4-fold increased risk of unprovoked venous thromboembolism (95% CI: 1.36-4.15) compared to men whose hematocrit was in the lower 40(th) percentile. The risk estimates were higher for men than for women both in continuous and category-based analyses. The findings for hemoglobin and red blood cell count were similar to those for hematocrit, whereas mean corpuscular volume was not associated with venous thromboembolism. Conclusions Our findings suggest that hematocrit and related hematologic variables such as hemoglobin and red blood cell count are risk factors for venous thromboembolism in a general population.