Carrier Testing and Who Should Be Tested

Carrier testing for Hemophilia is offered to assist a family in determining whether a biological female carries a variant within the factor VIII or factor IX gene. This may be offered because a new diagnosis of Hemophilia has occurred in an affected family member or maybe because the biological female is having symptoms herself. It is important to distinguish between testing of the factor level and genetic testing. As someone may have normal clotting factor levels but carry a pathogenic variant within the gene. Both should be performed in biologic females at risk to be carriers for Hemophilia. Genetic testing is the most predictive and effective way to identify the pathogenic variant that may exist in the gene and provide a diagnosis for the individual. 

Genetic testing (also known as molecular testing) evaluates the factor VIII and/or factor IX gene for variants that may be disorder-causing. 

Disorder-causing variants are unique changes within the genetic structure that impact how the gene will function. 

Within the Hemophilia community, we are aware of many variants that occur more commonly than others and this information can often assist healthcare providers with additional information about inhibitor risks. Additionally, knowing the specific variant within a family or an individual assists with pregnancy management and testing of other at-risk family members.

Genetic testing to determine one’s carrier status has become more accessible to individuals and is often covered by health insurance. Genetic testing is usually performed from a small amount of blood obtained from a vein in your arm (similar to other blood draws) or even from saliva or cheek swab.

An alternative way to evaluate whether someone may be a carrier for the condition is to test the amount of factor produced by the factor VIII or factor IX gene. The results of these tests are only informative if the factor level is less than we would expect (less than 50%). If the factor level is “normal”, this DOES NOT mean that the individual is not a carrier. Before genetic testing was as good as it is today, factor levels were the only way that we could check to see if someone was a carrier. However, this is an unreliable test as the results can be influenced by other factors such as pregnancy, certain forms of birth control, stress, exercise, and a recent or current illness. Therefore, it is ideal that testing of factor levels be performed when a biological female is in good health and not pregnant.

QUICK TAKE: Genetic Testing remains the gold standard for determining whether someone may be a carrier for Hemophilia. Genetic testing, however, may not be able to successfully identify all variants within the factor VIII and factor IX genes. For that reason, factor levels can be evaluated to assess the likelihood that someone is a carrier. Factor levels below 50% in biological females suggest that they are a carrier, but this is not diagnostic. Factor levels above 50% can be falsely reassuring.