Resistance to the anticoagulant effects of activated protein C (APC) was initially reported by Dahlback et al (1993) in a family with venous thrombosis. Protein C is an anticoagulant protein synthesised in the liver. Once activated on the endothelial cell surface APC selectively degrades the activated clotting factors Va and VIIIa. APC resistance (APCR) has been shown to be associated, in 90% or more of cases, with a mutation in the factor V gene (Arg506Gln) which gives rise to a variant form of factor V, factor V Leiden (FV Leiden). FV Leiden has normal procoagulant activity, but is partially resistant to cleavage and inactivation by APC. This gives rise to a hypercoagulable state.

FV Leiden is recognised to be the commonest of the inherited risk factors for VTE with a high prevalence in populations of European origin. The prevalence of the mutation ranges from 2-5% among European studies.

The phenotypic screening test for APCR used at HWL Laboratory is based on the prolongation of the activated partial thromboplastin time (APTT) by the addition of exogenous APC. The results are expressed as an APC ratio, the ratio between the APTTs measured in the presence and absence of added APC. The results for the patient are "normalised" to take into account day-to-day and run-to-run variations. The APC ratio of a normal patient will be approximately the same as the control (0.88-1.15). The normalised ratio of an APC-resistant patient will generally be <0.70.

A modified APTT-based screening test for APCR is used in many laboratories, including HWL Laboratory. In this modified method, plasma is prediluted in factor V deficient plasma to normalise clotting factor concentrations. This method is useful in anticoagulated patients and it has been shown to allow reliable screening for pre-existing APCR during pregnancy. The presence of a lupus anticoagulant may still give rise to anomalous results. The modified assay is highly sensitive and specific to the presence of th FV Leiden mutation.

The increased risk for thrombosis confered by the presence of FV Leiden has been estimated from The Leiden Thrombophilia study. Heterozygous carriers have an increased risk for thrombosis of up to sevenfold. Homozygotes have an 80-fold increased risk. Of major importance is the interaction of FV Leiden with other risk factors for thrombosis. Pregnancy, the use of oral contraceptives, and immobilisation are non-genetic risk factors which may be associated with increased risk of thrombosis in the APC-resistant patient. FV Leiden has been shown to present an additional risk factor in hereditary protein S deficiency, protein C deficiency and antithrombin deficiency.

Given, the high sensitivity and specificity of the modified APTT-based APC screening test, this test should be used as the first-line screening test. Positive results should be genotyped to confirm heterozygosity or homozygosity for the FV Leiden mutation. Homozygosity is associated with a high risk of venous thrombosis and accurate genotyping is essential for appropriate clinical management.