Carrier Testing for Cystic Fibrosis: Some straightforward answers to common questions

CF is a condition in which mucous is abnormally thick because of an abnormality in a gene known as the CFTR gene.  This results in digestive problems in infancy and chronic lung disease.  The average life expectancy for individuals with CF is 35 years.  Although life expectancy has increased as treatment advances have occurred, it remains a serious, chronic disease for those individuals affected.  The condition does not affect intelligence, but males with CF are infertile.

There are milder forms of CF, one of which causes absence of the vas deferens in men, but no lung disease.  Occasionally an individual with CF has very mild lung disease and few other medical problems.

CF is inherited as an autosomal recessive disorder.  This means that males and females are equally likely to be affected and that both parents must be carriers of CF to have an affected child.  Carriers have absolutely no manifestations of CF, and they have no health problems related to being a CF carrier.  Approximately 3-4% of the Caucasian population are CF carriers.  When both parents are CF carriers, there is a 25% chance in each pregnancy for the baby to be affected with CF.

• Anyone who has a relative with CF or whose reproductive partner has a family history of CF
• Any woman who is having prenatal diagnosis for other reasons (such as maternal age or for a risk for another genetic disorder)
• Anyone who is contemplating carrier screening for other genetic conditions
• Any European-Jewish woman or her European-Jewish partner
• Any Caucasian woman and/or her Caucasian partner
  

We at The Genetics Center feel a genetic consultation is very helpful if there is a family history of CF or if other prenatal testing is being considered.  Ideally the consultation should occur prior to conceiving or as early as possible after a pregnancy is recognized.

This is a difficult question, and current recommendations emphasize that is up to the individual to decide.  The ability to detect CF carriers is less than 100% and varies depending on a person's ethnic background.  Sensitivity ranges from less than 60% in Hispanics and Asians to 97% in persons of European-Jewish (Ashkenazi) ancestry.  In addition to the approximately 3% of Caucasians who are expected to be identified as CF carriers, another 3-5% will have variations of potential genetic significance that might, under certain circumstances, result in a child with mild cystic fibrosis or male infertility.  Testing is typically performed on a small sample of blood, but certain laboratories can alternatively use a sample of cells brushed from the inner cheek.

We suggest the following be considered when making a decision about CF carrier testing:

• Do you consider CF a serious enough condition that knowing whether a fetus is affected prior to the baby being born would be important to you?

Newborn screening will soon include CF.  With early identification, digestive problems related to CF can be effectively treated, and it is hoped that some of the lung problems may be prevented.  It is not presently possible to prevent the lung disease that eventually develops in children and young adults with CF, but more effective treatment may be available in the future.

• Do you feel comfortable with some uncertainty about test results?

For instance, if one member of a couple is a carrier and the other is not, depending on the ethnic backgrounds of the couple, there is still a chance (which may be higher than the 1/3,000 general population risk) of having a baby with CF.  Prenatal testing for CF would not be available in this situation.

• Are you and your partner of ethnic backgrounds where cystic fibrosis is a significant concern and where carrier testing is sufficiently sensitive?

For a couple of European-Jewish ancestry, testing is highly sensitive, since 97% of Ashkenazi Jewish CF carriers are correctly identified as being carriers.  For an Asian couple, not only is the disease much less common (1/32,000 in Asians as opposed to 1/3,000 in Caucasians), but carrier testing may detect as few as 1/3 of carriers.  Carrier testing in persons of European ancestry is 85-90% sensitive, and somewhat less sensitive when the ethnic backgrounds include Mediterranean or Southern European ancestry.  African-American couples have a 1/15,000 risk for having a child with CF, and carrier testing identifies 70% of carriers.

• Are you having (or contemplating having) prenatal testing for other reasons?

For instance, if you are already planning to have an amniocentesis because of maternal age, carrier testing for CF prior to the amniocentesis can be very reasonable, since fetal cells will be available for prenatal diagnosis of CF for "at risk" couples without additional risk to the fetus.

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.