The diagnosis of HB is established by assaying F9 activity, rather than by genetic testing. Given that F9 is a vitamin K-dependent protein, all patients with mild to moderate reductions in F9 activity should be evaluated to exclude vitamin K deficiency. Reduced F9 activity in at-risk females typically confirms HB carrier status. However, normal F9 activity does not exclude the possibility of being a carrier. In this circumstance, molecular genetic testing would be the only option for diagnosis of HB.
Knowledge of F9 genotype does not alter clinical management of HB patients; however, it may predict the risk of developing F9 inhibitors and anaphylaxis in response to F9 concentrate therapy.12 Although knowledge of the proband's genotype is important for carrier testing of at-risk female family members, the size of the F9 gene renders it more amenable to analysis for the specific F9 mutation causing HB.
Molecular Genetic Carrier Testing for HB
For a discussion applicable to molecular genetic carrier testing for HB, see the Clinical Utility of Testing section on HA.
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