Exchange between intravascularly and extravascularly injected radioiodinated fibrinogen and its in vivo derivatives.

To test the hypothesis that circulating FDP may stem from extravascular proteinolysis of fibrin (fibrinogen), eight patients with a pleural or peritoneal effusion had an intravenous injection of 125I-fibrinogen and a simultaneous intracavitary injection of 131I-fibrinogen. Two days after injection a mean of 12% of intravascularly injected radioactivity was found in the extravascular cavity, whereas 4.8% of the extravascularly administered radioactivity could be recovered in the circulation. Of the activity exchanged in both directions, more than half was protein-bound, and more than a quarter was clottable. The results demonstrate not only a rapid and considerable extravasation but also a definite back-flow of both clottable and unclottable fibrin (fibrinogen) derivatives from the extravascular space into the circulation. Thus, in the presence of pleural or peritoneal effusion, a shortened fibrinogen t1/2 as well as raised levels of FDP should be interpreted with caution. They may be due, at least to some extent, to an exchange between intravascular and extravascular spaces rather than to intravascular coagulation or fibrinolysis, respectively. The demonstration of fibrinogen proteolysis in effusions further emphasizes that conventional compartmental analysis of plasma and urine radioactivity after intravenous radiofibrinogen injection is highly questionable in pathological conditions.