Genetic testing is a procedure that screens a person’s genetic material (DNA) for the presence of mutations that might cause disease.
Cystic fibrosis (CF) is caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. Every person inherits two copies of this gene, one from each biological parent. CF is a recessive disorder, meaning that a person will only develop the disease if both of their copies of CFTR are mutated.
A person with just one mutated gene copy will not develop CF, but can pass the mutated gene on to their children. Such individuals are called carriers.
When is genetic testing done?
Genetic testing may be requested to diagnose CF, along with other diagnostic methods, such as a sweat test or an immunoreactive trypsinogen test.
This test also may be conducted to determine whether two people who want to have biological children are CF carriers, especially if one or both have a family history of the disease. Knowing whether or not a person or their partner is a carrier for CF can be of importance in family planning.
If two carriers await a child, a prenatal screening (testing the DNA of the developing fetus) can determine if their child will have CF.
Prenatal screening can be done by either amniocentesis (removal of amniotic fluid, which surrounds the fetus) or chorionic villus sampling — taking a sample of tissue from the placenta, a structure that provides oxygen and nutrients to the fetus. Amniocentesis is usually done between 15 and 20 weeks of pregnancy, whereas chorionic villus sampling can be done earlier in pregnancy.
Genetic testing may also be a part of CF newborn screening.
How genetic testing is performed
Genetic testing is done by analyzing genetic material (DNA) in a person’s cells. Usually, the cells are collected from blood or from inside the cheek. The sample is then sent to a lab, where DNA is isolated and checked for the presence of disease-causing mutations.
What the results mean
Test results must be interpreted in the context of the person’s signs and symptoms, physical examinations, medical and family history, ethnic background, and the results of other laboratory tests.
A positive result
A positive result in genetic testing of CFTR mutations is more than 99% accurate. However, the test cannot predict the degree of symptom severity; 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 CF symptoms, then the person is likely a carrier for CF. If a child is diagnosed with this disorder, then their relatives — especially biological siblings — should also be tested to determine their carrier status.
A negative result
A negative test result is not as accurate as a positive one: over 2,500 mutations in CFTR have been described so far, and there are rare disease-causing mutations that the test may not catch. If genetic testing produces a negative result, a person may still carry a mutated CFTR gene, with the chance of this happening depending on race or ethnic group, as well as how many mutations the given test screens for.
Genetic Counseling
The purpose of genetic counseling is to provide people with information about how their genetics may impact their health. A genetic counselor can work with patients to understand their family health history, and recommend genetic tests. The counselor can also help in interpreting test results, putting complex genetic information into context so that people can understand what it means for them and their families.
Genetic counselors can connect patients and families with needed resources (other healthcare providers, support groups, etc.), and often aid in family planning.
Because there are so many CF-causing mutations, genetic counseling for CF can be very complex. A genetic counselor needs to consider medical and psychosocial aspects while determining the best course of counseling for the patient.
Last updated: Sept. 16, 2021
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