Basics and Practice in Evaluating Plasminogen
Authors: Vinazzer H. 
Published: April 2009

There exist different ways of assays of plasminogen which give information about different properties of this proenzyme. The concentration of plasminogen can be determined by its antigenicity. Since the normal concentration of plasminogen in plasma is between 15 and 25 mg/dl the test can be carried out by simple methods such as radial immunodiffusion on Partigen® plates. The possibility of errors is small and there is no need of special apparatus. The disadvantages are the lapse of 24 h until the result is available and the fact that the knowledge of the concentration does not give any information about the activity. The activity can be measured by different coagulation tests. A typical assay would involve activation of plasminogen to plasmin, addition of plasminogen-free thrombin and measuring of the lysis time. The result is however, dependent on more than one variable. Plasmin is rapidly inhibited by alpha-2-antiplasmin (APL) and there is also a dependence of the lysis time on the amount of clottable fibrinogen in the test system. Better results can be obtained by the use of diluted test plasma and addition of a constant amount of plasminogen-free fibrinogen. A different way would be the use of the euglobulin fraction instead of plasma. This has however, the possible disadvantage of incomplete precipitation of plasminogen. Instead of coagulation tests the activity can also be determined when diluted activated plasma is placed on plasminogen-free fibrin plates and the amount of lysis in the plate is recorded. All assays of this group also depend on the method of activation of plasminogen. There may be different results when tests are simultaneously carried out after activation with streptokinase, urokinase and t-PA respectively. A different way of determining plasminogen activity is the use of chromogenic or fluorogenic substrates. These methods have different advantages. Some substrates are highly sensitive to the SK-plasminogen complex which is formed in plasma when SK is added in considerable abundance. This complex is not inhibited by APL and there is no fibrin involved in this type of reaction. The method is, therefore, more specific than most coagulation tests. The splitting of the substrate is measured photometrically as an endpoint test or as a kinetic reaction. For clinical purposes the determination of plasminogen activity by a chromogenic substrate method is sufficient. Only in order to detect abnormal plasminogen molecules an additional test of plasminogen concentration is a minimum requirement which is then supplemented by attemps of activation with different substances such as urokinase and t-PA. Plasminogen assays in clinical practice are used for different reasons. Congenital deficiency or a defective plasminogen molecule with increased thrombotic tendency have been described but are very rare. Acquired deficiency of plasminogen is found in severe liver dysfunction, in DIC and during therapy with SK or UK. In the latter cases assays of plasminogen activity are or greater clinical importance and should, therefore be available in laboratories of larger hospitals.