Carrier testing in haemophilia is dependent on the accurate taking of a family history and confirmation of family relationships. A woman can be defined as an obligate (definite) carrier of haemophilia if; her father has haemophilia or she has two children with haemophilia or she has a child/grandchild with haemophilia and a maternal relative with haemophilia. It is important to remember that in around 30% of woman who are carriers of haemophilia there is no family history of the disorder and it has occurred in the family as a result of a spontaneous mutation. The genetic abnormality (mutation) that causes haemophilia occurs on the X chromosome which is passed from parents to their children.
Normal factor VIII levels are between 50 – 150%. Females who are carriers of haemophilia may have reduced factor VIII or factor IX levels and approximately 10% of carriers of severe haemophilia will have levels below 30%. Mild deficiencies of factor VIII or factor IX may only cause bleeding problems at the time of surgery, dental extraction or after major trauma. It is important to measure the factor VIII or factor IX level of any child or woman who is an obligate carrier or a potential carrier of haemophilia. This test should ideally be carried out within the first year of life but because of problems obtaining blood samples from very small children, this may be delayed until the second year of life. However, it is essential that the factor VIII/factor IX level is measured immediately in children undergoing surgery or in children with bleeding symptoms.
If it is discovered that a carrier does have a low level of factor IX or factor VIII then treatment options should be discussed. Carriers with reduced factor VIII or factor IX levels are managed in a similar manner to affected males with mild haemophilia. It is often necessary for carriers to have treatment with factor before a surgical procedure, giving birth or after a traumatic accident. Factor VIII levels increase during pregnancy. However, factor IX levels do not usually increase with pregnancy.
Why do Carrier Testing?
A. To allow women and families to be informed of the risk of having an affected male child.
B. By excluding someone from being a carrier, they can be reassured about their pregnancy and health of any male child.
C. By diagnosing someone as being a carrier we are informing them that any male child would have a 50/50 chance of having haemophilia, and that this would be a lifelong disorder.
D. If your daughter is a carrier, this will allow her to educate herself about haemophilia and prepare for the prospect of having children with haemophilia.
Frequently asked questions
What is the chance of a woman who is a known carrier passing on haemophilia to her children? If a known carrier has a male child then there is a 50% chance that the boy will have haemophilia. If the carrier has a female child then there is a 50% chance that the girl will be a carrier.
What is the chance of a man with haemophilia passing on haemophilia to his children? If a man with haemophilia has a male child then this boy will not inherit haemophilia. If the child is female then this girl will be a carrier.
How is carrier testing carried out? The method that is used at present to determine carrier status is direct mutational analysis. A blood sample is first required from an affected male in the family and from the female requesting testing. There are many different genetic mutations that result in haemophilia and the most common of these are carried out first. The identification of more unusual mutations may be time-consuming and could take up to 2 years for results to be available.
What does it involve for me if I want to be carrier tested? An affected male in the family needs to attend with you at clinic or at an earlier date to give a blood sample. You will have an opportunity to discuss the genetic inheritance of haemophilia and the consequences of being a carrier of haemophilia with one of the specialist nurses. You will be asked to sign a consent form and a blood sample will be taken. You will be contacted when the results of your carrier testing are available and asked to attend to discuss these results and to give a confirmation sample of blood.
If I am a carrier of haemophilia with a low factor level can I bleed? Females who are carriers of factor VIII or factor IX deficiency may have reduced levels. Approximately 10% of carriers will have a level below 30% (normal is 50 – 150%). Carriers with reduced factor VIII or factor IX levels are managed in the same way as men with mild haemophilia, they will have a bleeding tendency which will be problematic at times of surgery, childbirth, dental work or after an accident or fall.
If a girl has a normal factor level does this mean that she is not a carrier? No. It is very important to note that most carriers of haemophilia will have a normal factor level.
I am a carrier of haemophilia and have one child who has haemophilia. Does this mean that the next child will not have haemophilia? No. The risk of having a child with haemophilia is the same for every pregnancy.
Diagnosis: Haemophilia Carriers
Prior to diagnosis your personal bleeding history will be taken. The following information will be sought:
- Do you bruise easily?
- Have you bleeding post dental or post operatively?
- Your menstrual history?
- Do you experience bleeding or bruising after trauma?
For carriers or possible carriers it is advisable to undergo genetic counselling preferably before becoming pregnant. Genetic counselling will provide you with information to enable you to make an informed decision in relation to family planning. It will also be an opportunity to address issues that you or your partner or family have about passing on haemophilia to your child.
Genetic Testing – The Process
- A member of the family who has haemophilia should initially give blood samples.
- Blood samples are taken from the person being tested.
- DNA is extracted and the sample is screened for the mutation.
- The result is given to the person who was tested, results in extreme cases may take 1 – 2 years
- A repeat sample is taken for confirmation.
- The patient is registered on the NCC database.
How can I be tested to see if I am a Carrier?
By contacting the National Coagulation Centre on 01 4162141 / 01 4162142 for advice and appointment.
Please also note:
- A member of your family with or carrying haemophilia must initially give blood samples.
- The Clinic is coordinated by Clinical Nurse Specialist.
- Results may take 1 – 2 years.
- A follow-up appointment is sent when results are available.
- A confirmatory sample is requested when attending for results of the initial sample.
Don’t forget you can contact us at the Society on 01 6579900 for further information on any of the above.