The Prothrombin Time: The Mechanism Of In Vitro Activation

We have described shortening of the prothrombin time (PT) when blood is collected in borosilicate or siliconized boro- silicate tubes. The shortening is time dependent and occurs more rapidly at 4° C than at room temperature. We have studied the mechanisms of in vitro activation of the PT utilizing normal blood and blood congenitally deficient in coagulation factors. We have found that the blood deficient in factor XI has the same rate of shortening of the PT as in normal blood, while blood deficient in factor IX has only 50/ that of normal and factor XII deficient blood does not have any in vitro activation. The blood deficient in Cl INH ihas the most severe shortening of the PT.Analysis of the coagulation factors affected during the in vitro activation of normal blood revealed that factor VII-X is increased and this is related primarily to factor VII activation. In borosilicate factor V did not change, factor IX decreased by 15-20% while factor XII showed no change in the assay system. The activation can be totally blocked by addition of Cl INH or corn trypsin inhibitor (specific inhibitor of XII activation) to whole blood. These studies indicate that the PT is shortened in vitro by at least two mechanisms of activation of whole blood: 1) factor XII is activated by surface contact with siliconized borosilicate or borosilicate and in turn, directly activates factor VII; 2) factor XIIa activates prekallikrein to kalli- krein which in turn activates factor IX. Factor IXa then activates factor VII and it would appear that approximately one-half of the activation of the PT is directly through this pathway. Inhibitors of factor XII activation, Cl INH and CFI can totally inhibit the activation in vitro. Thus, surface activation of factor XII is the pivotal reaction in the in vitro shortening of the PT in whole blood. These studies allow new strategies for the prevention of in vitro activation of the PT, and may allow for more precise measurement of the PT without concern for contact activation by the use of specific inhibitors.