Top Toolbar


Types of Carrier Tests

What Kinds of Tests Are Used?

Genetic counseling
With so many options, hemophilia carrier families considering their first or subsequent child benefit enormously from genetic counseling, which is available through most Hemophilia Treatment Centers (HTCs). Highly trained genetic counselors provide extensive information and support for carriers and their families prior to actual carrier testing, before they conceive a child, and after the unpredicted birth of a son with hemophilia. Carrier testing is a very personal decision.

Types of Tests

  • Family tree. The first step in finding out whether a woman is a hemophilia carrier is getting a detailed family history. As part of genetic counseling, a physician or genetic counselor will take a family history and draw a family tree, called a pedigree chart. In general, the pedigree chart contains information on 3 generations: children and cousins; parents, aunts, and uncles; and grandparents. The pedigree helps identify people within the family who could be carriers of the gene
  • Factor Levels. Blood tests are performed to measure levels of clotting factors VIII (8) and IX (9). Factor level tests are 70% to 90% accurate in determining a person’s carrier status. On average, carriers have lower levels of factor VIII (8) or IX (9) in their blood than women who are not carriers. Although carriers have only one normal X chromosome, their clotting factor levels aren’t half of the normal level. Indeed, normal covers a wide range of clotting factor levels. Carriers and noncarriers often have similar clotting factor levels. In addition, age, health status, pregnancy, nursing, and medications, including birth control pills, can affect clotting factor levels. Knowing that a woman’s clotting factor level is low can help doctors provide the best care in situations involving bleeding, such as surgery or dental procedures. Female carriers with clotting factors that are less than 50% of normal can also have menorrhagia and joint bleeds
  • DNA Testing. The most accurate way to tell whether a woman is a carrier is to examine her DNA for genetic changes that cause hemophilia. Questions concerning a certain method or laboratory test should be referred to a genetic counselor, but a few basics are below
    There are 2 types of DNA analysis available:
    • Direct Mutation Testing
      For hemophilia A and B, it is possible to look for mutations within the factor VIII (8) and factor IX (9) genes, respectively. This approach is called direct mutation testing and is the most accurate method for identifying carriers. A blood sample from the male family member with hemophilia is checked first. In about 98% of cases, a mutation can be identified

      Next, a blood sample from the woman desiring carrier testing is obtained and her DNA is checked for the specific mutation. If the mutation is found in the female’s DNA, she is a carrier; if the mutation is not found, then in most cases she is not. (In rare instances, a woman’s eggs but not her blood cells have the mutation)

      Even in families with no living males, direct detection methods may provide helpful information. Sometimes a mutation that is known to occur in other individuals with hemophilia is found, suggesting that a woman may be a carrier. This testing is performed at specialized laboratories. Typically, results are available in a few weeks

    • Linkage (Indirect Testing)
      In some cases of hemophilia A and hemophilia B, a gene mutation cannot be identified. Using indirect testing (called linkage testing) may help determine the gene carrier status of females by tracking the gene in the family. To perform this test, blood samples are taken from the male with hemophilia and other family members. Patterns of linked DNA in the person with hemophilia are compared to the DNA in family members to check for the same pattern. Linkage testing is not as accurate as direct testing and does not provide information for all families. A genetic counselor can discuss the benefits and limitations of this type of testing for a specific family
  • Prenatal Testing
    In families with a history of hemophilia, prenatal testing is an option to see if the fetus is affected. This testing can be done early in the pregnancy, allowing the family to make informed decisions and preparations. Such information can help prepare the parents emotionally, as well as to talk with the doctors about plans for delivery. Some tests require a small sample of the amniotic fluid, placenta, or blood from the umbilical cord. If you are pregnant and you think you could be a carrier, or if you have one child diagnosed with hemophilia and you are expecting again, talk with your obstetrician about what precautions should be taken to ensure a safe delivery for you and your child
  • Tests for von Willebrand Disease (VWD)
    Several tests can be used to look for mutations in the von Willebrand factor (vWF) gene. These mutations are, however, much more likely to be present in the more severe forms of VWD than the milder ones

    A family practitioner, internist, obstetrician/gynecologist, or pediatrician can refer a person interested in learning if they have a vWF gene mutation to a hematologist or Hemophilia Treatment Center (HTC) for testing. Because not all laboratories are able to perform these tests, traveling to get tested is likely

For more information, go to MASAC Guidelines #197