Two kinds of laboratory tests can be performed for carriers of hemophilia: factor assays and genetic tests. Factor assays measure the amount of clotting factor in a person’s blood. While this information is useful, some carriers will have normal clotting factor levels. Therefore, this test has the potential to provide falsely reassuring or incorrect information to women who may indeed be carriers; it cannot be used to confirm whether a woman is a carrier of hemophilia. Factor levels can vary significantly among family members. For example, a woman with very low factor levels can have a daughter whose levels are near normal. Factor assays should therefore be done for each known or suspected carrier within a family. Stress, inflammation, infections, certain medications, birth control pills, and pregnancy can all cause factor VIII levels to rise and therefore affect test results. Factor VIII levels also tend to go up as people get older.Genetic tests such as mutation analysis look directly for the altered gene that’s responsible for hemophilia. This is the only way to be absolutely sure that a woman is a carrier. Information obtained from these tests is also more revealing for other family members. However, genetic tests can be costly and may not be available in all centres.
source: http://www1.wfh.org/publication/files/pdf-1471.pdf
