Resistance to activated protein C and factor V Leiden.

Over the last four years, there has been an explosion of knowledge about APCr and factor V Leiden. However, there remain a considerable number of difficult clinical areas in which there are no clear answers. Undoubtedly, factor V Leiden is commonly found in association with venous thromboembolic disease in whatever manifestation, but equally it has an unusually high frequency in the general population. Only a small proportion of those that carry the mutation develop a thrombosis. It is estimated that only 6% of those that carry the mutation will develop a thrombosis over a 30-year period, whilst for antithrombin, Protein C or Protein S deficiency, this figure is nearer 60%. Particular areas of difficulty remain in relation to the use of the combined OCP and in the management of the asymptomatic carrier of the mutation in pregnancy. Although the scientific basis of APCr and factor V Leiden is well established, its natural history remains relatively poorly understood, probably as a consequence of its relative novelty. Despite the plethora of new data that have appeared, there remains much to be learnt about factor V Leiden and the APCr phenotype.