What is Carrier Testing and Who Should be Tested?
There are two ways to test a woman to see if she is a carrier of hemophilia.
The first way is to test the factor level in her blood. Women who carry the hemophilia gene may have a level that is lower than normal. Some carriers may have levels low enough to cause bleeding problems.
The blood tests for factor level can tell if a woman is a carrier 80% to 90% of the time. Since the tests are not perfect, you also look at the family history of hemophilia. These two pieces of information are used to tell a woman her chances of carrying the hemophilia gene.
This type of carrier testing is not done when a woman is pregnant, nursing a baby, or taking birth control pills. These things can mess up the test results.
The second way to tell if a woman is a carrier is through DNA testing. DNA is the stuff that genes are made of. For the DNA test, DNA is taken from the woman's blood and checked for the hemophilia gene. The DNA results and the family history of hemophilia are used together to provide the answers.
The DNA test is almost always right. Small amounts of blood are needed from a family member with hemophilia, parents of the person being tested, and sometimes from other relatives like grandparents or siblings. It may take up to three months to get the test results. However, pregnant women can get the results faster. Your hemophilia treatment center has more information about DNA tests and blood tests for carriers.
Some women do not have to be tested to know that they are carriers. These women are definitely carriers:
- daughters of men with hemophilia
- mothers who have more than one child with hemophilia
- mothers who have a child with hemophilia and another relative with hemophilia
When a mother has only one son with hemophilia and no other family history of it, she may or may not be a carrier. A mutation may have happened in the gene. This could have happened in the mother, in one of her ancestors, or in the son with hemophilia himself. If the mutation happened in the son, the mother is not a carrier. Other children she may have in the future are not at risk of having hemophilia. The son with hemophilia, of course, can pass the gene on to his children.
These women might be carriers and could benefit from testing:
- sisters of a person with hemophilia
- aunts, nieces, and girl cousins of a person with hemophilia (but only those on his mother's side of the family).