Lower activated partial thromboplastin times are associated with higher levels of some coagulation factors and may represent a procoagulant tendency.

In the Atherosclerosis Risk in Communities study, we studied the 13-year risk of venous thromboembolism in relation to baseline activated partial thromboplastin time in 13,880 individuals. We also studied 258 venous thromboembolism cases and 589 matched controls with measurements of additional coagulation factors.

After adjustment for demographics and procoagulant factors reflected in the activated partial thromboplastin time (fibrinogen, factors VIII, IX, and XI, and von Willebrand factor), participants in the lowest 2 quartiles of activated partial thromboplastin time compared with the fourth quartile had 2.4-fold (95% confidence interval [CI], 1.4-4.2) and 1.9-fold (95% CI, 1.1-3.2) higher risks of venous thromboembolism. The risk associated with activated partial thromboplastin times below the median was higher for idiopathic (odds ratio 5.5; 95% CI, 2.0-15.5) than secondary venous thromboembolism (odds ratio 1.74; 95% CI, 0.88-3.43). Subjects with both activated partial thromboplastin times below the median and factor V Leiden were 12.6-fold (95% CI, 5.7-28.0) more likely to develop venous thromboembolism compared with those with neither risk factor (P interaction<.01). A lower activated partial thromboplastin time also added to the thrombosis risk associated with obesity and elevated D-dimer.

A single determination of the activated partial thromboplastin time below the median increased the risk of future venous thromboembolism. Findings were independent of coagulation factor levels, and a low activated partial thromboplastin time added to the risk associated with other risk factors.