Section 10 - Cystic Fibrosis Clinical Care Pathway

5.4 Carrier testing of siblings
The healthy siblings of a child with CF each have a two in three risk of being
a carrier of CF. As a general rule, the appropriate time to arrange for brothers
or sisters to be tested is in the mid to late teenage years when they can
understand the implications of the test. At the present time, CF carrier
testing is not offered to minors in this clinic, as it is not considered to be
ethically acceptable to do genetic testing on individuals without their
informed consent.
Occasionally, parents who do not wish to have prenatal diagnosis, request
that a new baby is tested immediately after delivery, so that they can start
appropriate treatment early and not be kept in suspense for some weeks
awaiting the results of a sweat test. The parents are informed as to whether
the baby is affected, unaffected or a carrier.
5.5 Prenatal screening
Parents of a child with CF have a one in four risk of recurrence. Some may
opt to have prenatal diagnosis, and a termination of pregnancy (TOP) if the
baby is predicted to be affected. Many couples find this a very difficult
subject, especially with the improvements in treatment and life expectancy.
Some feel that having a TOP implies that they would not have had their
affected child had they been aware of the diagnosis during pregnancy. Other
couples do not agree with termination on religious or personal grounds, and
may wish to discuss their risks and other possible options (e.g. donor
insemination, pre-implantation diagnosis, etc.) in order to avoid having a
further affected child. For this reason, all parents who are considering having
further children should be offered genetic counselling.
During pregnancy, a mother may have a routine fetal ultrasound scan which
may reveal an anomaly that increases the risk that the fetus may be affected
by CF. In these cases, both partners are offered prenatal screening. The results
of this will allow further evaluation of the risk that their baby may be affected
and provide information regarding possible prenatal diagnosis.
Prenatal diagnosis is usually carried out by testing a chorion villus sample
(CVS) not before 11 weeks’ gestation. The test has about a one percent risk
of causing a miscarriage. The result usually takes about a week to process so
that a first trimester termination can be performed if necessary.
It is always best if the preliminary gene typing has been carried out on the
family prior to pregnancy, so that the laboratory knows precisely which
mutations they are testing for in the CVS. Even if the affected child has an
undetectable mutation, it is still possible to do a prenatal diagnosis (using
gene tracking), providing the work-up has been done prior to pregnancy.
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