[Activated protein C resistance as a cause of thrombophilia]

The proportion of identifiable causes of familial thrombophilia has increased from 5-10% to 60-70% since the identification of activated protein C resistance (aPCR) in February 1993 by Dahlbäck et al. A mutation in the factor V gene (G-->A, 1691) leads to the so called Leiden mutation (R 506 Q) that produces a mutated factor V resistant to the catalytic action of activated protein C (aPC), yet normal in its procoagulant properties. This recently identified aPCR is in Nordic populations the most prevalent and well defined genetic defect associated with disease so far described. Its prevalence in the general population ranges from 0% to up to 15% and suggests that a positive genetic selection pressure has been involved. The aPCR phenotype can be assessed in vitro by measurement of the prolongation of the activated partial thromboplastin time in the presence of aPC, whereas the aPCR genotype is studied using polymerase chain reaction searching for the Arg to Gln mutation in the coagulation factor V gene. Some acquired conditions such as the presence of lupus anticoagulants, antiphospholipid antibodies, pregnancy, liver disease and contraceptives may lead into the aPCR phenotype. The aPCR search must be the initial step in the study of a patient with thrombophilia, either inherited or acquired aPCR together with protein C, protein S and antithrombin III explain 60 to 70% of cases of familial thrombophilia.