Protein C response to induction and withdrawal of oral anticoagulant treatment

Protein C activity and antigen levels have been related to clotting activities of factors VII and X during the induction and withdrawal periods of oral anticoagulant treatment. Both factor VII and protein C activities fell rapidly during induction but factor VII showed a more rapid and much more marked depression than protein C. In contrast, reductions in factor X were much slower. Protein C antigen, although depressed rapidly at the initiation of treatment, did not subsequently fall to the same degree as protein C activity. The ratio of activity to antigen became progressively smaller. On discontinuation there was a reversal of the pattern but with two important differences. Firstly, there was evidence of an excessive rise ('rebound') of factor VII compared with the steady state levels in these patients; and secondly there was a surprisingly slow return of protein C to normal levels after the oral anticoagulant was withdrawn (levels were still below normal on day 4). These observations lend support to gradual withdrawal of oral anticoagulants after a period of long-term administration. The results suggest that after discontinuation of long-term anticoagulants patients may have increased coagulability up to four days.