The most severe forms of haemophilia typically affect males, while females are conventionally designated as “carriers.”

Carriers often do not show symptoms of haemophilia because, although they have an abnormal F8 or F9 gene on one X chromosome, the other X chromosome contains a functioning F8 or F9 gene, which generally manages to produce clotting factor levels within the normal range.

A percentage of carriers have low Factor VIII (FVIII) or Factor IX (FIX) activity due to of Lyonization (i.e., random deletion of one of the two X chromosomes, also called X-inactivation), which can result in mild, moderate, or even severe haemophilia in rare cases. Symptomatic females should be designated as having haemophilia of a specific severity, as is the case for males.

Carriers with FVIII/FIX levels in the normal range may never require replacement therapy. However, some carriers with factor levels in the lower than normal range experience bleeding problems similar to males with mild haemophilia, as well as female-specific issues such as prolonged or profuse menstrual bleeding.

For these reasons, the World Federation of Hemophilia recommends that female carriers of haemophilia, regardless of factor level, should be registered with a haemophilia treatment center and that those with low factor levels should be treated and managed like males with haemophilia.



  • WFH Guidelines for the Management of Hemophilia, 3rd edition, Chapter 9: Specific Management Issues, Windyga et. al, Haemophilia. 2020;26(Suppl 6):1–158., DOI: 10.1111/hae.14046
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Learn more about the meaning of the words you read on this page and learn about the entire glossary on hemophilia.

A solid mass constituted by a clump of platelets held together by a mesh constituted by a protein called fibrin.

The function of the clot is to plug the opening that forms in the wall of a damaged blood vessel, in order to prevent blood loss.

Once the damage has been repaired, the clot is dissolved by specific mechanisms triggered by our body.

A group of proteins with enzymatic functions involved in the clotting process.

All clotting factors are assigned a Roman numeral, although each one also has a proper name.

Some are produced by the liver, whereas others are synthesised by the endothelial cells that make up the inner lining of the blood vessels.

A protein belonging to the clotting factor category, a group of enzymes involved in the blood-clotting process.

It is encoded by a gene on the long arm of the X chromosome.

Factor VIII is also known as anti-haemophilic factor (AHF).

Product containing a high concentration of freeze-dried factor VIII, used for replacement therapy in the treatment of haemophilia A.

A ereditary genetic illness, characterised by a deficiency of clotting factor VIII, that exposes the individual to a greater risk of both internal and external bleeds.

Haemophilia A is more common in males, whereas females tend to be healthy carriers of the condition.

The typical symptoms of the condition include haemarthroses (joint bleeds) and haematomas (muscle bleeds).

Condition that occurs when factor VIII activity is between 1% and 5%.

Condition that occurs when factor VIII activity is between 1% and 5%.

Condition that occurs when factor VIII activity is <1%.