[Characteristics of the fibrinolytic system in the newborn].

Recent development of microassays and determination of age-specific normal ranges have shed new light on the components and functioning of the neonatal fibrinolytic system. Plasminogen and tissue plasminogen activator levels are low in neonates, who generate plasmin more slowly and in smaller amounts than adults. Quantitative and qualitative changes occur as the fibrinolytic system matures. This is also true of the coagulation system responsible for the production of thrombin, which is the target for plasmin. These data are essential to assess the risk of thrombosis in neonates and, if appropriate, to guide management decisions including selection of a thrombolytic agent, of the optimal dosage, and of the best laboratory tests for monitoring purposes. Ongoing studies are investigating the mechanisms involved in neonatal lysis of thrombin clots occurring naturally or as the result of thrombolytic therapy.