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The severity of bleeding in a haemophiliac is related to the level of the relevant factor in the blood. One ml of normal plasma contains one unit or 100% Factor activity.
Patients with less than 1% activity have a serious, possibly life-threatening, diasthesis. Commonly, first presentation is at about 6 months when the infant starts to crawl.
Bleeding may follow trauma or occurs spontaneously. Haemarthroses and soft tissue bleeds are common, and if treated inadequately may produce joint deformity and crippling. Bleeding from the gut and the renal tract occur infrequently. Cerebral haemorrhage is unusual but is the most common cause of death. Spontaneous bleeds are probably traumatic events which would otherwise be normally tolerated. The patient's routine is greatly affected by the frequency and unpredictability of the bleeds.
Patients with less than 5% show severe bleeding following injury.
Those with more than 5% activity show minor symptoms, whilst those with more than 25% activity, although strictly haemophiliac, often remain undiagnosed unless tested.