The origin of fibrin breakdown products and the interpretation of their appearance in the circulation.

It has been shown that the incubation of human plasma with urokinase at a concentration sufficient to cause rapid lysis of the clots formed on the addition of thrombin does not give rise to the production of measurable concentrations of non-clottable fibrinogen breakdown products. Also, breakdown products could not be detected in the course of experiments in vivo when urokinase was administered to monkeys and only in very low concentrations when a fibrinolytic state was induced by exercise in three healthy human volunteers. In contrast, high concentrations of breakdown products were found after thrombin infusion into monkeys. It is concluded that circulating fibrinogen is not readily broken down into non-clottable products by the fibrinolytic enzymes, and that normal animals and healthy human subjects do not have substantial deposits of fibrin that are available for breakdown during a fibrinolytic episode. The presence of breakdown products in the circulation is therefore likely to be indicative of the fibrinolytic response to an initial coagulation event.