Thrombin generation capacity is enhanced by low antithrombin activity and depends on the activity of the related coagulation factors

Background Supplementation with antithrombin (AT) concentrates is now common in the treatment of congenital and acquired AT deficiency. However, there is no established consensus on the target and timing of supplementation. We aimed to elucidate the effects of AT deficiency on the balance between coagulation activation and inhibition using a thrombin generation assay as in vitro global assay. Methods Samples were prepared by admixing commercially acquired AT-deficient plasma with < 1% AT activity with pooled normal plasma. The AT activity in each sample was adjusted to 100, 90, 70, 50, 40, 30, 10, 5, and < 1%. A thrombin generation assay was performed in each sample. AT concentrate-spiked samples were also prepared by adjusting the AT activities in four types of the concentrates: one recombinant and three plasma-derived AT concentrates. The final targeted AT activities in the samples were adjusted to 100, 50, 30, and 5% by spiking each concentrate into the AT-deficient plasma. We also prepared samples with five levels of prothrombin time (PT) % in coagulation factors with the AT activity fixed at 30% by dilution by mixing AT-deficient plasma and normal plasma with Owren's veronal buffer to adjust the coagulation factor activities in several proportions. The theoretical target PT% values were 100, 66, 50, 40, and 30%. A thrombin generation assay was performed on all samples. Results The ability to generate thrombin depended on the AT activity, and the amount of thrombin generation was increased as AT was decreased. Additionally, the amount of thrombin generation was changed significantly when AT activity was <= 50%, indicating that AT suppressed thrombin generation. In particular, thrombin generation was remarkable when AT activity was < 30%, and it can be assumed that the prognosis is poor due to organ failure from thrombotic tendency. Conclusions The results presented in this basic research were found to be consistent with the clinical findings to date. The mechanism by which 30-50% of AT activity is set as the clinical boundary was elucidated by the thrombin generation assay.