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Summary. Clinically observed excellent hemostasis in neonates despite low levels of clotting factors is not completely understood so far. Therefore, we investigated whether physiological low levels of the inhibitor protein C (PC) facilitate thrombin formation in tissue factor (TF)-activated plasma samples. PC was activated by endogenously generated thrombin after addition of soluble thrombomodulin (TM). The capability of activated PC (APC) to suppress thrombin formation was significantly more pronounced in adult than in cord plasma. Addition of 4 nM of TM decreased the thrombin potential (TP) in cord plasma by 10%, and in adult plasma by 52% in the presence of 5 pM TF. We demonstrate that this low anticoagulant action of PC is attributable to the low levels of tissue factor pathway inhibitor (TFPI) and antithrombin (AT) physiologically present in cord plasma. Addition of 4 nM TM decreased the TP by 58% in cord plasma adjusted to contain TFPI and AT at adult levels in the presence of 5 pM TF. Thus, the combined low anticoagulant action of the three inhibitors APC, TFPI, and AT in cord plasma allows enhanced thrombin formation associated with shorter clotting times compared with adult plasma when low amounts of TF are applied to initiate clot formation. Although our laboratory experiments do not allow definite conclusions for various clinical situations, our data might contribute to explain excellent hemostasis in neonates despite low levels of procoagulants.

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