Fibrinogen present in EDTA--anticoagulated plasma stimulates the tissue-type plasminogen activator-catalysed conversion of plasminogen to plasmin.

The presence of soluble fibrin in plasma is an early and sensitive indicator of activation of the coagulation system. Quantitative spectrophotometric assays for soluble fibrin can be based on the principle that soluble fibrin stimulates the tissue-type plasminogen activator-catalysed conversion of plasminogen to plasmin. It was previously shown that treatment of purified fibrinogen by EDTA, which removes the three tightly bound Ca2+ ions, results in exposure of tissue-type plasminogen activator-catalytic sites similar to those unveiled by thrombin. Since EDTA is a common anticoagulant, it was of interest to study the effect of EDTA on a test based on plasminogen activation. It is concluded that the determination of soluble fibrin in EDTA-anticoagulated plasma from healthy individuals gives a false positive indication of the presence of soluble fibrin. This was true irrespective of whether the test was performed at pH 7.4, 7.8 or 8.5. The most probable explanation is that tissue-type plasminogen activator-stimulating sites are exposed in fibrinogen by EDTA. Therefore, EDTA-plasma is unsuitable for assaying soluble fibrin with tests based on the tissue-type plasminogen activator-mediated conversion of plasminogen to plasmin.