Measurement of Spontaneous Fibrinolytic Activity.

The spontaneous fibrinolytic activity of blood is due to a labile activator which appears to be stabilized by adsorption to fibrin (Fearnley and Tweed, 1953; Fearnley, 1953; Flute, 1960). At present activator activity can be measured only indirectly through its conversion of plasminogen to plasmin which in turn causes fibrinolysis. Whatever method is used it is essential to cool the blood sample to 0°C. immediately it is obtained, and to carry out any subsequent manipulations at this temperature in order to preserve the activator which disappears rather rapidly from fluid blood kept at room temperature. Two factors inhibit spontaneous fibrinolysis, the natural inhibitory capacity of the blood, which is probably complex, and the presence of calcium (Fearnley and Tweed, 1953; Fearnley and Lackner, 1955). The first can be diminished by dilution as shown by MacFarlane (1937) or by separation of the euglobulin fraction, the inhibitors remaining in the supernatant. Calcium can be removed by precipitation. It is easier to measure fibrinolytic activity in blood than in plasma, and we have developed two simple methods for this purpose, one of which is really a modification of the other.