Low tissue factor pathway inhibitor (TFPI) together with low antithrombin allows sufficient thrombin generation in neonates.

Neonates have an excellent hemostasis despite, in comparison to adults, markedly decreased and delayed ability to generate thrombin. Only 30-50% of peak adult thrombin activity can be produced in neonatal plasma by means of conventional in vitro assays. We show that in contrast to conventional activation, activation with small amounts of lipidated tissue factor (<10 pmol L(-1)) results in shorter clotting times and faster activated factor X- and thrombin generation in neonates compared with adults due to the concomitant action of low tissue factor pathway inhibitor and antithrombin. The concentrations of both inhibitors in cord plasma are approximately 50% of the respective adult values. After addition of 2.5 pmol L(-1) lipidated tissue factor, cord plasma clotted approximately 90 s earlier than adult plasma and the amount of free thrombin generated was approximately 90% of adult value (291 +/- 14 vs. 329 +/- 16 nmol L(-1) min(-1), P < 0.01). Our results might help to explain the clinically observed excellent hemostasis of neonates despite low levels of procoagulant factors.