How does the amount of factor VIII or factor IX deficiency affect hemophilia severity?

In the context of hemophilia, the severity of the disease is directly correlated with the level of deficiency of the required coagulation proteins. Specifically, the greater the deficiency of either factor VIII or factor IX, the more severe the clinical manifestations of the disease will be.1

While hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency) are distinct conditions, their signs and symptoms are clinically indistinguishable. In both cases, individuals with severe deficiencies are prone to spontaneous bleeding into joints, which can lead to chronic arthritis, as well as dangerous bleeding into the brain or abdominal cavity. Because these factors are essential components of the intrinsic pathway of blood coagulation, their absence prevents the effective formation of a fibrin clot, thereby increasing the risk of uncontrolled bleeding.12