Translate this website into the following languages:

Close Tab
UC San Diego Health
menu iconMenu
search iconSearch

Patient Education: About Hemophilia

Hemophilia is a genetic disorder in which blood does not clot normally. Hemophilia is characterized by the absence of certain clotting factors. In Hemophilia A, Factor VIII is deficient; in Hemophilia B, it is Factor IX. Patients with hemophilia have life-long bleeding problems.

Hemophilia can be inherited, or occur spontaneoulsy. It almost exclusively affects males. Females can be carriers, and only develop the disease in rare cases. 

With severe hemophilia, percent clotting factor activity is less than 1% of normal. In moderate and mild hemophilia, clotting factor activity levels are 2%-4%, and 5%-30%, respectively. Clotting factor levels of greater than 30% are usually sufficient to provide adequate blood clotting. 

While patients with severe hemophilia have life-long spontaneous bleeding into joints, muscle and skin, patients with milder forms of hemophilia may only bleed with injury, trauma or accidents. In fact, patients with mild hemophilia may not know they have the disease until bleeding after an injury or surgery.


Hemophilia is caused by the absence of certain clotting factors. (In Hemophilia A, the clotting factor called Factor VIII is deficient; in Hemophilia B, it is Factor IX.) It affects approximately 1 in 5,000 to 10,000 males.

The disease is called “X-linked,” meaning that the genes for these factors are on the X chromosome. Because they have two X chromosomes, females are usually only carriers of the disease (one healthy X chromosome usually provides a sufficient amount of clotting factors). The disease usually affects only males, whose chromosomes are XY.

Females can develop hemophilia under extremely rare circumstances. The average female carrier with 2 X-chromosomes has half normal levels of factor VIII (Hemophilia A) or factor IX ( Hemophilia B). These levels are sufficient for normal hemostasis. However, sometimes the second healthy X-chromosome is disproportionally inactivated or missing. This can lead to hemophilia in females. (See our pages on Services for Carriers or Bleeding Disorders in Women.)


Treatment for hemophilia consists of administering clotting factor products, to replace the clotting factors that are missing because of the inherited condition. Clotting factors can either be manufactured by recombinant technologies, or be purified from donor plasma. Treatment by infusion can be prophylactic several times per week, or on-demand when bleeding occurs or is anticipated (for example, before a sporting event).

Not all patients with hemophilia “bleed the same,” and it is therefore up the patient and doctor to decide which treatment strategy and dose are best. Treatment by infusion can be prophylactic several times per week, or on-demand when bleeding occurs or is anticipated (for example before a sporting event). Most patients learn in childhood how to self-infuse at home with adequate amounts of clotting factors and supplies stored wherever they go. This avoids unnecessary trips to the Emergency Department and delays in treatment.

The Hemophilia and Thrombosis Treatment Center clotting factor program, Total Hemophilia Care, provides individual home infusion services to our patients. Every patient seen at our center can join this program. By federal mandate, the proceeds of the factor sales will come back to the Hemophilia Treatment Center and patient care.


The most devastating long-term complication in hemophilia is the development of inhibitors against infused Factor VIII-or Factor IX-preparations. Inhibitors are antibodies that neutralize the activity of a clotting factor. The incidence of  inhibitors is approximately  20%-30%. Inhibitor development typically occurs within the first few months after start of clotting factor administrations.  Inhibitors can be treated by using various immune tolerance treatment protocols. Treatment can take up to 2 years, but is successful in 70%-80% of patients.  In order to treat bleeding efficiently during that time, and for non-responders to immune tolerance, so called “bypassing agents” have to be used. Those are clotting factor concentrates that contain activated FVII, and can to some extent bypass the clotting pathway intrinsic to Factor VIII and Factor IX.

Hemophilia Carriers

The average female carrier of hemophilia with 2 X-chromosomes has half the normal level of factor VIII (Hemophilia A) or factor IX ( Hemophilia B). However, factor activity levels can be higher or lower than 50%. This depends on the pattern of so-called X-chromosome inactivation. Usually each female randomly inactivates half of all her X-chromosomes since only one X-chromosome is needed. This process is called "lyonization," after the scientist Mary Lyon, who discovered this process. If an early female embryo inactivated non-randomly most of the X-chromosome with the normal factor gene, then the woman’s factor level can be lower that 30% of normal. Thus, the woman may manifest with hemophilia. Vice versa, if an early female embryo inactivated non-randomly most of the X-chromosome with the hemophilia gene, the woman’s factor levels can be normal. It is therefore not reliable to base the diagnosis of hemophilia carrier status solely on clotting factor activity, and we offer chromosomal gene mutation analysis to identify the mutation. For more information, see Services for Carriers.

Our bimonthly newsletter delivers healthy lifestyle tips, patient stories and research discovery news. Subscribe: