🧭 When to suspect

Haemophilia is an inherited bleeding disorder in which the blood fails to clot normally because of a deficiency of a single clotting factor. Haemophilia A is a deficiency of factor VIII (FVIII) and accounts for around 80–85% of cases (roughly 1 in 5,000 male births); haemophilia B (Christmas disease) is a deficiency of factor IX (FIX) and accounts for most of the remainder. Because FVIII and FIX act together in the same part of the clotting cascade, the two are clinically indistinguishable – only a factor assay tells them apart.

Inheritance is X-linked recessive, so the condition overwhelmingly affects males and is carried by females. Around one-third of cases arise from a new (spontaneous) mutation, so a negative family history does not exclude it. Carrier females may themselves bleed abnormally if their own factor level is low.

Suspect haemophilia in a boy (or a man not previously investigated) with bleeding that is disproportionate to the injury: easy or extensive bruising as a child becomes mobile, prolonged bleeding after circumcision, venepuncture, dental work or surgery, recurrent or hard-to-stop nosebleeds, or bleeding into a joint (haemarthrosis) or muscle. A bleeding history in male relatives on the mother's side is an important clue. Severity – and therefore management – is determined almost entirely by the residual factor level.

Severity	Residual factor activity	Typical bleeding pattern
Normal	50–150% (0.50–1.50 IU/mL)	No abnormal bleeding
Mild	5–40% (0.05–0.40 IU/mL)	Bleeding usually only after surgery, dental extraction or major trauma; often diagnosed late
Moderate	1–5% (0.01–0.05 IU/mL)	Bleeding after minor trauma; occasional spontaneous bleeds
Severe	< 1% (< 0.01 IU/mL)	Frequent spontaneous bleeding into joints and muscles

Source: UKHCDO · British Society for Haematology

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