Anticoagulant effects of APC in normal plasma are enhanced by protamine. (A) Increasing concentrations of APC (1.25-20 nM) were added to normal pooled plasma in the presence (●) and absence (○) of 30 μg/mL protamine, and plasma APTT was measured. (B) Thrombin generation in normal pooled plasma (○) was compared with plasma spiked with 1 nM APC (▲) alone, 30 μg/mL protamine (□) alone, or 1 nM APC plus 30 μg/mL protamine (●) in combination. (C) ETP (% of normal pooled plasma) was calculated after titration of protamine (0.2-80 μg/mL) into normal plasma containing 1.0 U/mL UFH and either in the absence (○) or presence (▼) of 1 nM APC.