Abnormalities in the gene encoding Factor VIII (FVIII) results in hemophilia A, an X-linked recessive bleeding disorder with a prevalence of 1 in 5000 males. Hemophilia A patients are classified into 3 different categories based on residual FVIII activity compared to normal: mild (6-40%), moderate (1-5%) and severe (<1%). This categorization correlates to some degree with bleeding phenotype, but does not completely define it. Some patients with hemophilia A bleed less often than others despite identical plasma FVIII levels. The cause(s) of this phenotype heterogeneity in hemophilia A remains largely unknown, despite a number of studies of possible factors.

Activated platelets, in addition to their role in primary hemostasis, play a major role in secondary hemostasis (coagulation) by providing a phospholipid surface to which coagulation factors bind. A role for platelets in the hemorrhagic propensity of hemophilia A has been suggested in the past, but only a small number of studies have been performed with limitations in assays performed and numbers of patients. The purpose of the present study is to determine whether platelet reactivity in severe hemophilia A patients is associated with past bleeding frequency and/or predicts future bleeding frequency.

Inclusion Criteria:

• Patients with severe hemophilia A who are being prophylactically treated with FVIII.
• Age of at least 2 years.
• Bleeding history of at least 6 months.
• IRB-approved informed consent.

Exclusion Criteria:

• Presence of FVIII inhibitors.
• Greater than 7 days since active bleeding.

Patients with severe hemophilia A who do not have inhibitors against FVIII and who have a bleeding history of at least 6 months.