Inherited thrombophilia due to activated protein C resistance is now r
ecognized as one of the major genetic risk factors in the development
of venous thromboembolic disease. Activated protein C resistance is se
condary to a point mutation in the factor V gene, factor V Leiden. The
high prevalence of this mutation in the general population, mainly in
Caucasians of European descent, is a major contributing factor to the
high incidence of venous thromboembolic disease in the United States,
affecting one in 1000 individuals annually. Heterozygosity and homozy
gosity for factor V Leiden increase the risk for thrombosis 5- to 10-f
old and 50- to 100-fold, respectively, compared with genotypically nor
mal individuals. Factor V Leiden is more common than all other known g
enetic risk factors for thrombosis, and its presence results in a comp
ounded risk in patients with simultaneous inherited abnormalities such
as protein C, protein S, antithrombin III deficiencies, hyperhomocyst
einemia, and/or acquired risk factors. Therefore, detection of activat
ed protein C resistance and genotyping for factor V Leiden are importa
nt for establishing risk for thrombosis and ultimately for patient man
agement.