"Normal" thrombin generation

We have investigated the influence of alterations in plasma coagulation fac tor levels between 50% and 150% of their mean values for prothrombin, facto r X, factor XI, factor IX, factor VII, factor VIII, factor V, protein C, pr otein S, antithrombin III (AT-III), and tissue factor pathway inhibitor (TF PI) as well as combinations of extremes, eg, 50% anticoagulants and 150% pr ocoagulants or 50% procoagulants and 150% anticoagulants in a synthetic "pl asma" system. The reaction systems were constructed in vitro using purified , natural, and recombinant proteins and synthetic phospholipid vesicles or platelets with the reactions initiated by recombinant tissue factor (TF)-fa ctor VIIa complex (5 pmol/L). To investigate the influence of the protein C system, soluble thrombomodulin (Tm) was also added to the reaction mixture . For the most extreme situations in which the essential plasma procoagulan ts (prothrombin, and factors X, IX, V, and VIII) and the stoichiometric ant icoagulants (AT-III and TFPI) were collectively and inversely altered by 50 %, a 28-fold difference in the total available thrombin generated was obser ved. Variations of most of these proteins 50% above and below the "normal" range, with the remainder at 100%, had only modest influences on the peak a nd total levels of thrombin generated. The dominant factors influencing thr ombin generation were prothrombin and AT-III. When these 2 components were held at 100% and all other plasma procoagulants were reduced to 50%, there was a 60% reduction in the available thrombin generated. No increase in the thrombin generated was observed when the 150% level of all plasma procoagu lants other than prothrombin was evaluated. When only prothrombin was raise d to 150%, and all other factors were maintained at 100%, the thrombin gene rated increased by 71% to 121%. When AT-III was at 50% and all other consti tuents were at 100%, thrombin production was increased by 104% to 196%. The additions of protein C and protein S over the 50% to 150% ranges with Tm a t 0.1 nmol/L concentration had limited influence on thrombin generation. In dividual variations in factors VII, XI, and X concentrations had little eff ect on the duration of the initiation phase, the peak thrombin level achiev ed, or the available thrombin generated. Paradoxically, increases in factor IX concentration to 150% led to lowered thrombin generation, while decreas es to 50% led to enhanced thrombin generation, most likely a consequence of factor IX as a competitive substrate with factor X for factor VIIa-TF. Red uctions in factor V or factor VIII concentration led to prolongations of th e initiation phase, while the reduction of TFPI to 50% led to shortening of this phase. However, none of these alterations led to significant changes in the available thrombin generated. Based on these data, one might surmise that increases in prothrombin and reductions in AT-III, within the normal range, would be potential risk factors for thrombosis and that algorithms t hat combine normal factor levels may be required to develop predictive test s for thrombosis. (C) 1999 by The American Society of Hematology.