What is genetic testing
Genetic testing is a procedure that screens a person’s genetic material (DNA) for the presence of mutations that might cause disease.
In the case of cystic fibrosis (CF) — a genetic disorder characterized by the buildup of thick mucus in various organs — genetic testing provides information about whether a person carries a mutation in the CFTR gene.
When genetic testing is ordered
Genetic testing for CF may be ordered in these cases:
- If a person has symptoms of CF, such as very salty sweat or frequent respiratory infections. In such cases, genetic testing is used as a diagnostic test for CF.
- If results after initial diagnostic tests, which can include a sweat test or immunoreactive trypsinogen (IRT) test, are positive. In such cases, genetic testing is used to confirm a CF diagnosis.
- If a close relative has been diagnosed with CF. In this case, genetic testing may provide information about whether an individual is a CF carrier, i.e., carries a single copy of the mutated CFTR gene. This information may be useful when planning a family.
- If a baby is conceived by parents who are known to be carriers, they are at risk of passing on the gene mutation. In such a case, prenatal screening (testing the DNA of the unborn baby) can be done by either amniocentesis (removal of amniotic fluid, which surrounds the unborn baby) or chorionic villus sampling (taking a sample of cells from the placenta, which connects the mother and the baby).
- If a baby is born to parents who are known to be carriers, the baby can undergo newborn screening for the presence of mutations in the CFTR gene.
How genetic testing is performed
Genetic testing is done by taking genetic material from cells, which are collected from a blood, saliva, or tissue sample from inside the cheek. The DNA, isolated from the sample, is checked for the presence of known mutations in the CFTR gene.
What the results mean
The test results must be interpreted in the context of the person’s signs and symptoms, physical examination results, medical and family history, ethnic background, and results of other laboratory tests.
A negative result
If genetic testing produces a negative result or reveals only a single copy of the mutated gene but the individual has signs and symptoms of CF, then further genetic testing or laboratory tests are recommended. In some cases, the person may have a rare genetic mutation in the CFTR gene that had not been previously identified.
A positive result
A positive result for genetic testing is more than 99 percent accurate and means that the individual has two copies of a mutated CFTR gene. However, the test cannot predict the degree of severity of the symptoms; different patients with the same mutation may have very different symptoms.
If genetic testing reveals only a single copy of a mutation and the person does not have any symptoms of CF, then the person is likely a carrier for CF. If someone is identified as a carrier, then their siblings may also be tested to determine their carrier status.
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