SUMMARY

Background: Acquired resistance to activated protein C has been reported during oral contraception and pregnancy, and may reflect a prothrombotic state. Methods: The thrombogenic potential of acquired APC resistance was studied in 41 neurosurgical patients who were enrolled in the placebo group of a thromboprophylaxis trial. Normalized activated protein C sensitivity ratio (nAPC-SR), clotting activity of factors V and VIII, and levels of protein C antigen were measured prior to and at days 3 and 7 after surgery. Bilateral venography was done in all patients at days 8-10 to demonstrate deep vein thrombosis. Results: A lowered nAPC-SR was found in 76 % (baseline), 80 % (day 3) and 88 % (day 7) of patients. It was inversely related to factor VIII clotting activity (p=0.0003) and protein C antigen (p=0.02). Deep vein thrombosis was demonstrated in 30 % of patients with a normal nAPC-SR and in 23 % of patients with a lowered nAPC-SR. Pulmonary embolism was not observed. Multivariate analysis did not identify a lowered nAPC-SR as a thrombotic risk factor, in contrast with gender (women, p=0.02) and Quetelet index ($25 kg/m2, p=0.006). Conclusions: Our data provide no evidence that acquired activated protein C resistance