Carrier Testing

Types of Tests Used to Identify Carriers

The types of tests that have been used for Glossary Link carrier testing include CK testing, muscle biopsy, and genetic carrier testing.  We recommend genetic carrier testing because it is very accurate and requires only a blood sample.  

In most cases, CK testing and muscle biopsy are not good choices for carrier testing. CK levels are higher in child and adolescent carriers than in adult female carriers, who are the ones more likely to have carrier testing. CK levels also may be increased for reasons other than muscular dystrophy, such as strenuous activity or sickness. 

A muscle biopsy is less accurate than genetic testing and it is an invasive test (it requires removing a piece of the muscle so it can be studied under a microscope).

A Woman Has a Child With DMD/BMD.  How Likely Is It That She Is a Carrier?

If you have a child with DMD or BMD and you have other affected male family members, for example an affected brother or nephew, it is extremely likely that you are a carrier.

If you have no other affected family members, there is a 66% (or 2 in 3) chance that you are a carrier. Approximately 33% (or 1 in 3) of cases of Duchenne muscular dystrophy are caused by what are called new mutations. These are random changes to the genetic code in the dystrophin Glossary Link gene that happen in only one egg or sperm. That one egg or sperm could create an affected male; rarely, an affected female; or a carrier female child who could later have affected children.

The possibility for new mutations is one of the reasons why 1/3 or more of individuals with Duchenne muscular dystrophy will have no family history. Another possibility is that some families have several generations with mostly or all females (that is, they have no boys to express the disease). These families may not know that there are several generations of carriers.

What is the chance of DMD/BMD happening again in a family?

A carrier has a 50% (1 in 2) chance that each of her sons will be affected by Duchenne or Becker muscular dystrophy and a 50% (1 in 2) chance that each of her daughters will be a carrier.

Women with germline Glossary Link mosaicism have a lower chance than a carrier to have children with muscular dystrophy, but their chance is higher than the chance in the general population. It is impossible to know the exact chance for a woman with germline mosaicism because there is no way to tell how many of her egg cells have the dystrophin Glossary Link mutation.

The only way to know for sure if a woman is a carrier is if she has a positive genetic carrier test.

If a woman has not had a genetic carrier test, her risk for having an affected son can be estimated by looking at her family history. Consider asking for a referral to a genetics professional, who is trained to use the family history to determine risk.

Affected people in the same family almost always have the same mutations in the dystrophin gene and will have the same type of muscular dystrophy.

What is Important to Know about Carrier Genetic Testing?

This section includes information on why genetic carrier testing is important, who might want to consider carrier testing, when carrier testing should be done, how carrier testing is done, and what the results mean.

Why is genetic carrier testing important?

Genetic testing can help tell whether a woman is definitely a carrier or whether she is very unlikely to be a carrier.

Carriers have an increased chance of having boys with Duchenne or Becker muscular dystrophy. If a woman knows she is a carrier, she can make more informed childbearing plans.

Carriers have an increased chance of heart problems. If a woman knows she is a carrier, she should be evaluated regularly by a doctor who knows about heart symptoms in carriers.

Identifying carriers in the family can provide information to other family members about their chance of also being carriers and having affected sons.

Who should consider carrier testing?

All women who could be carriers, based on their family history, should be offered carrier testing. This includes women with:

  • sons or brothers with DMD/BMD,
  • uncles or cousins on their mother’s side of the family who have DMD/BMD,
  • mothers or sisters who are carriers for DMD/BMD, and
  • aunts or cousins in their mother’s side of the family who are carriers for DMD/BMD.

Carrier testing can give women information about their chance of having affected sons, and about their own health.

When should carrier testing be done?

Carrier testing should be offered to at-risk, adult women.

It is important to consider the maturity of the woman who is offered carrier testing to make sure she understands the information. Carrier testing is generally not offered to children.

For more information, click here to read an article written for health care providers about issues involved with using genetic testing in minors.

How is carrier genetic testing done?

Carrier testing is done on a blood sample.  There are different lab methods that can be used for carrier testing. The method for carrier testing should be determined by the woman’s family situation, including whether the mutation in the family is known and her family history.

If the mutation in the family is known, only that mutation needs to be tested.

If the mutation in the family is not known because the affected person has not had testing, it is best to test him first, by blood or using a frozen muscle biopsy sample.

If the mutation in the family is not known and there are no living relatives to test, a comprehensive testing plan can still find most (but not all) carriers.

If genetic testing was done in the past and no mutation was found, it might be appropriate to test the affected individual again using new and improved tests, which can identify more mutations.

What do results from carrier testing mean?

If carrier testing finds a mutation, the woman is a carrier.

If the mutation in the family is known and carrier testing is done to find that same mutation, but the lab does not find the mutation in the woman- that means that the woman is very unlikely to be a carrier.

If the mutation in the family is not known and appropriate carrier testing does not find any type of mutation, it reduces the chance that the woman is a carrier, but does not make the chance zero.  This is the most difficult type of result for healthcare providers and patients to interpret.  An experienced genetics professional can help women understand this result and what additional testing might be useful.