Clinical Utility of Testing

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.

Molecular Genetic Diagnostic Testing of the Proband

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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