The Blood-Clotting Factors

Coagulation TestsThe reagents and causes of prolongation of the one-stage (Quick) prothrombin time are listed in Table 5. The critical level of fibrinogen below which the prothrombin time is prolonged is probably about 100 mg. per 100 ml. of plasma. Heparin may interfere at a number of points in the coagulation mechanism (as discussed below under "Anticoagulants"), and a blood specimen for a valid determination of prothrombin time should be drawn at least four and preferably six hours after the last dose of intravenously administered heparin. The prolongation of the prothrombin time in cases of congenital deficiency of prothrombin itself . . .