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Haemophilia A and haemophilia B

Haemophilia A is a heritable bleeding disorder resulting from low or, in severe cases,  absent functional levels of coagulation factor VIII. Inheritance is X-linked recessive. Haemophilia A is the most common of the severe bleeding disorders, with an approximate incidence of 1 new case per 5,000 male births. Haemophilia A is caused by mutations in the factor VIII (F8) gene.

Haemophilia B is a heritable bleeding disorder resulting from low or, in severe cases, absent functional levels of coagulation factor IX. Inheritance is X-linked recessive. Haemophilia B has an approximate incidence of 1 new case per 25,000 male births. Haemophilia A is caused by mutations in the factor IX (F9) gene.

Investigations carried out include familial mutation identification and interpretation, carrier diagnosis and prenatal diagnosis (PND).

Genetic PND may be performed early in pregnancy, usually via chorionic villus sampling (CVS) at 11 to 13 weeks of gestation. This allows first trimester diagnosis, avoiding late termination of pregnancy. The large majority of patients, following appropriate expert counselling, do not opt to terminate an affected pregnancy.

Genetic PND may also be performed in the third trimester of pregnancy, usually at about 36 weeks of gestation, by means of amniocentesis. This approach enables diagnosis of the presence or absence of a severe bleeding disorder in the fetus, facilitating important decisions about the clinical management of childbirth.

Genetic diagnosis of the causative mutation can provide important clinical information about the risk for inhibitor development associated with clotting factor replacement therapy. This may particularly be valuable in mild or moderate haemophilia A.

Knowledge of the causative mutation may influence diagnosis and clinical management in specific cases, for example genetic diagnosis of haemophilia B Leyden, where the onset of puberty leads to amelioration of the haemophilia phenotype, or the identification of mutations associated with a 1-stage/2-stage clotting factor VIII assay discrepancy in haemophilia A.