Measurement of a newly developed thrombomodulin addition activated partial thromboplastin time assay in patients with deep venous thrombosis.

We developed a simple assay using rabbit thrombomodulin (TM) based on an activated partial thromboplastin time method, which detected the response to TM in plasma coagulation. We call it thrombomodulin addition clotting time (TACT). The anticoagulant response to TM was calculated by dividing the clotting time with TM by the clotting time with buffer solution. Results were expressed as TACT ratio, which indicates the degree of inhibition of plasma clotting by TM. Using this assay, we measured the TACT ratio in 80 patients with deep-vein thrombosis (DVT) and in 126 controls matched to the patients according to age and sex. A significant difference in the TACT ratio was observed between patients with DVT (mean 1.874) and controls (mean 1.956) (p < 0.001). Twenty- three patients (29%) had TACT ratios below the 10th percentile (1.757) of distribution of control subjects (odds ratio: 3.5; 95% confidence interval (CI): 1.7-7.2). After excluding subjects with a deficiency of protein C, protein S and antithrombin III, we found an odds ratio for DVT of 3.4 (95% CI: 1.6-7.2). These data suggest that natural anticoagulant deficiencies do not influence the TACT ratio, and our case-control study may show that the plasma of patients with DVT has a low response to TM.