The levels of tissue plasminogen activator (t-PA) antigen, PA activity, plasminogen activator inhibitor 1 (PAI-1) antigen, PAI activity and t-PA·PAI-1 complex were measured in normal plasma or serum obtained by different methods. PA activity could be measured only in acidified plasma samples. Plasma and serum t-PA antigen levels were equivalent. PAI activity and PAI-1 antigen levels elevated during incubation of citrated blood at 37°C for one hour. Marked elevations of the levels of PAI-1 antigen, PAI activity and t-PA·PAI-1 complex were found in serum. These findings indicate that prolonged storage of anti-coagulated blood leads to the release of PAI-1 from platelets, and that some of the released PAI-1 are active. However, no change was found in plasma levels of PAI-1 antigen, PAI activity and t-PA·PAI-1 complex if citrated blood samples were centrifuged immediately at 4°C either when antiplatelet agents were added for blood collection or not. Therefore, the levels of PAI-1 antigen, PAI activity and t-PA·PAI-1 complex would be determined precisely in the routinely used citrated plasma if citrated blood is centrifuged immediately after blood collection.
