The majority of patients with hemophilia have a known family history of the condition. However, about one-third of cases occur in the absence of a known family history. Most of these cases without a family history arise due to a spontaneous mutation in the affected gene. Other cases may be due to the affected gene being passed through a long line of female carriers.
If there is no known family history of hemophilia, a series of blood tests can identify which part or protein factor of the blood clotting mechanism is defective if an individual has abnormal bleeding episodes.
The platelet (a blood particle essential for the clotting process) count and bleeding time test should be measured as well as two indices of blood clotting, the prothrombin time (PT) and activated partial thromboplastin time (aPTT). A normal platelet count, normal PT, and a prolonged aPTT are characteristic of hemophilia A and hemophilia B. Specific tests for the blood clotting factors can then be performed to measure factor VII or factor IX levels and confirm the diagnosis.
Genetic testing to identify and characterize the specific mutations responsible for hemophilia is also available in specialized laboratories.