The dynamics of clot formation using either thrombin or arvin to clot purified human fibrinogen F XIII free fibrinogen and human plasma was studied using the thrombelastograph (TEG). Varying concentrations of these components including calcium were observed for their effects on the overall clotting patterns. The action of plasmin or urokinase on these systems was also evaluated. The production of equal area thrombelastographic tracings required over four times the amount of Ancrod compared to thrombin in the absence of calcium. Calcium ions exerted profound effects upon the TEG configuration inhibiting clot formation at certain concentrations in the Ancrod system, while enhancing the rate and size of fibrin formation in the thrombin systems. These changes were heavily influenced by the ionic strength of the buffer system, and appear related to factor XIII activation. No TEG differences were observed when thrombin or Ancrod formed clots were subjected to proteolysis in the absence of calcium. Enhancement of Ancrod but not thrombin clots subjected to proteolysis in the presence of calcium was observed with early Ancrod samples, but this phenomenon was minimal with more purified Ancrod solutions available recently. Doubling of the control area was demonstrated in some cases with the less pure Ancrod and appeared to be related to the dose of urokinase employed. If one assumes that the lytic agents preferentially attack the residual fibrinogen molecule rather than formed fibrin then greater amounts of fibrin monomer could result and produce a larger initial clot than with Ancrod alone. Repeating these experiments with F XIII free fibrinogen produced the same results. Enhancement effects disappeared when the proteolytic agent was added after clotting had begun, and increased lytic susceptibility of the impure Ancrod systems sometimes occurred under these circumstances. Ancrod and thrombin formed fibrin clots exhibit different characteristics depending upon the nature of the test system. The potential thrombogenic properties of the earlier contaminated Ancrod samples emphasized the importance of using only the pure preparations for therapy in man.
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