The severity levels of hemophilia are determined by the level of residual/remaining clotting factor (factor VIII or factor IX) activity in the patient’s blood, which is denoted by the percentage of factor activity in the blood (or international units per ml of blood – IU/ml).

According to the World Federation of Hemophilia, the normal range of factor activity is between 50% and 150% percent (0.50-1.5 IU/ml).

Depending on the severity level, hemophilia is classified as mild, moderate or severe.

Mild hemophilia

The clotting factor level in the blood of a patient with mild hemophilia is between 5% and 40% (0.05-0.40 IU/ml). In these cases, bleeding typically occurs only after a trauma, such as surgery or major injury. Otherwise, patients either bleed rarely or may never have a bleeding problem. About 25% of people with hemophilia have the mild form of the disease.

Moderate hemophilia

Patients with moderate hemophilia have between 1% and 5% (0.01-0.05 IU/ml) of clotting factor activity in their blood. They bleed infrequently (perhaps once a month). Prolonged bleeding tends to occur following trauma such as minor surgery or injury, and dental treatment, for example. Spontaneous bleeding episodes, although rare, may occur in patients with this type of hemophilia. About 15% of people with hemophilia have the moderate form of the disease.

Severe hemophilia

Patients with severe hemophilia have less than 1% (0.01 IU/ml) clotting factor activity in their blood. They bleed frequently (sometimes once or twice a week). Bleeding may occur spontaneously into their muscles and joints. Prolonged bleeding occurs following trauma, injury, or surgery, but also may occur spontaneously for no obvious reason.

Variations within severity levels of hemophilia

Regardless of the severity level, the frequency and type of bleeds may vary from one patient to the other. For instance, 10% to 15% of patients with severe hemophilia A have less frequent spontaneous bleeding episodes. This variation in bleeding symptoms within the same severity level may arise due to various reasons, such as the type of mutation within the blood clotting factor genes, mutations in other clotting factor genes, inaccurate clotting factor measurement relative to overall clot formation, variations in the immune system, and environmental factors such as physical activity.

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