Genetic testingThe discovery of the CFTR gene in 1989 allowed the development of an accurate genetic test for CF. Genes from a small blood or
tissue sample are analyzed for specific mutations; presence of two copies of the mutated gene confirms the diagnosis of CF in all but a very few cases. However, since there are so many different possible mutations, and since testing for all of them would be too expensive and time-consuming, a negative gene test cannot rule out the possibility of CF.
Couples planning a family may decide to have themselves tested if one or both have a family history of CF. Prenatal genetic testing is possible through amniocentesis. Many couples who already have one child with CF decide to undergo prenatal screening in subsequent pregnancies, and use the results to determine whether to terminate the pregnancy.
Siblings in these families are also usually tested, both to determine if they will develop CF, and to determine if they are carriers, to aid in their own family planning. If the sibling has no symptoms, determining his carrier status is often delayed until his teen years or later, when he is closer to needing the information to make decisions.
What role does genetic testing play?
Cystic fibrosis is an autosomal recessive disorder, which means a copy of a mutated gene must be inherited from each parent in order for a baby to be born affected with the disease.
People can carry the defective gene without knowing it - more than 10 million Americans do, including one in every 29 whites (which translates into about 1 in every 3500 Caucasian babies born affected with CF). Because there are so many unsuspecting carriers, most babies with the disease are born into families that did not know they were at risk. If both parents
harbor the defective gene, they have a one-in-four chance of having a baby with CF.
When should testing be performed?
Testing is best performed prior to a woman getting pregnant. If both parents are determined to be carriers, they may opt for in vitro fertilization, where the resulting embryos can be tested for the disease and only healthy ones are implanted into the mother's uterus.
A test typically costs about $265 (as of 2001), and physicians say many insurers will cover it. Every patient should check with their health insurance carrier prior to testing to determine whether they will be responsible for payment.
How accurate is the test?
The test is good but not 100 percent accurate. There are about 1,000 known mutations in the cystic fibrosis causing gene, and the new guidelines advise test laboratories to check for a minimum of the 25 most common mutations. Genzyme Corporation, the largest test provider, typically tests for 87 mutations, which
rules out greater than 90% of Caucasian mutations. How well this widespread gene testing works will influence how other gene tests are introduced to Americans.
How can I get more information?
Your obstetrician should have written information on CF carrier testing, either from one of the many laboratories that perform the testing or publications from the American College of Obstetricians and Gynecologists. If desired, your obstetrician can refer you to a geneticist for more comprehensive consultation. The
geneticist, after obtaining a complete family history and reviewing relevant medical information, will be able to discuss the issue in more detail, arrange the actual carrier testing if desired, interpret the results of testing, and make recommendations for further testing and/or prenatal diagnosis.
Should my spouse and I get tested to see if were carriers of mutated gene?
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