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How is hemophilia diagnosed?

Hemophilia A and B are diagnosed by measuring factor clotting activity. Individuals who have hemophilia A have low factor VIII clotting activity. Individuals who have hemophilia B have low factor IX clotting activity.

Genetic testing is also available for the factor VIII gene and the factor IX gene. Genetic testing of the FVIII gene finds a disease-causing mutation in up to 98 percent of individuals who have hemophilia A. Genetic testing of the FIX gene finds disease-causing mutations in more than 99 percent of individuals who have hemophilia B.

Genetic testing is usually used to identify women who are carriers of a FVIII or FIX gene mutation, and to diagnose hemophilia in a fetus during a pregnancy (prenatal diagnosis). It is sometimes used to diagnose individuals who have mild symptoms of hemophilia A or B.

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