Diagnosis of two related carriers of severe haemophilia B with no family history

Haemophilia B is an X-linked disease affecting 1 in 30 000 males. Carrier diagnosis is usually carried out only in female relatives of haemophilic males, and the likelihood of discovering a carrier without a haemophilic male is very low. In this report we present the cases of two related women without a family history of haemophilia who were diagnosed as haemophilia B carriers. Following a minor haemorrhage in the proband, she and her mother were thought to be haemophilia B carriers because of a low factor IX level (16 and 23 IU dL^–1, respectively; normal values >50 IU dL^–1). The non-sense mutation C31118T, which is associated with severe haemophilia B, was detected in both women. This allowed us to diagnose them as being definite carriers of severe haemophilia B and give appropriate genetic counselling.