Identification of causative mutations in the factor VIII gene (X chromosome) for families with hemophilia A can permit very accurate determination of carrier status of at-risk females in these families and provides options for prenatal diagnosis. To identify the mutation in a hemophilia A family, a blood sample is required from an affected individual (proband). Because inversions in intron 22 of the factor VIII gene are found in approximately 45% of patients diagnosed with severe hemophilia A, the intron 22 gene inversion analysis is initially performed on samples from patients with severe hemophilia A. For cases of mild or moderate disease or for those with severe disease and not having a gene inversion, Comprehensive Mutation Analysis (CMA) can be performed. This analysis entails full gene sequencing of the factor VIII gene and, if negative, further analyses for the factor VIII gene intron 1 inversion and von Willebrand Factor Normandy mutations. The intron 1 inversion is present in about 5% of patients with severe hemophilia A.
After a mutation is detected in a proband, carrier testing and prenatal diagnosis may be offered to the family. In cases where a proband is not available for testing, analysis may be performed on a sample from an obligate carrier.