Factor X and its deficiency states.

Factor X is one of the vitamin-K-dependent serine proteases. As a result of its position at the convergence of the intrinsic and extrinsic pathways of the clotting cascade, it plays a crucial role in blood coagulation. Factor X interacts with components of both pathways of coagulation, leading to its activation and the formation of the prothrombinase complex. The gene for factor X has been cloned and sequenced and maps to the long arm of chromosome 13, approximately 2.8 kb downstream of the factor VII gene. Each of the exons of factor X encodes a specific functional domain within the protein. In terms of its gene structure and amino acid sequence, factor X shows significant homology with other vitamin-K-dependent clotting factors, suggesting an origin in some common ancestral protein. Factor X deficiency is one of the rarest of the inherited coagulation disorders. Such deficiencies are inherited in an autosomal recessive manner and are characterized by a variable bleeding tendency. In its homozygous form, factor X deficiency has an estimated prevalence of 1:500 000 but in its heterozygous form it has an estimated frequency of ∼1:500 although affected individuals are often clinically asymptomatic. Acquired deficiencies of factor X are uncommon and in isolation are seen most frequently in patients with amyloidosis and in association with upper respiratory tract infections. Treatment of the deficiency state involves factor X replacement with either fresh frozen plasma or prothrombin complex concentrates. However, the latter may be associated with an increased risk of thrombosis.