By Dr Ananya Mandal, MD
Cystic fibrosis is often diagnosed at birth, through the routine screening of newborns. The disease may also be suspected early on in childhood due to clinical symptoms such as a persistent cough, recurrent chest infections and excessively salty sweat.
Newborn screening for cystic fibrosis
All newborn babies are screened for cystic fibrosis. A small amount of the baby’s blood is drawn from the baby’s heel and transferred to a special card that is sent to a laboratory to be analysed. This blood is tested for levels of immunoreactive trypsinogen. This is a pancreatic enzyme precursor that is elevated in the blood of babies with cystic fibrosis due to blocked pancreatic ducts failing to drain adequately.
If cystic fibrosis is suspected, genetic testing is performed to look for the mutation that causes this disease. The mutation occurs in the gene that codes for a protein called the cystic fibrosis transmembrane conductance regulator (CFTR).
Diagnostic sweat test
Another test used to check for cystic fibrosis is the sweat test, which measures the amount of chloride present in a patient’s sweat. For collection of the sweat, a chemical that causes sweating is placed on a small area of the arm or leg and an electrode is placed over the area. A weak electrical current is then applied, which causes sweating and a mild tingling, warm sensation. After five minutes, the current is stopped and the electrode removed. The sweat is collected on a piece of filter paper or gauze and sent for analysis. High levels of chloride in the sweat indicates cystic fibrosis.
X-rays of the chest may be performed to view the lungs and assesses any damage caused by recurrent infections and mucosal blockage. A computed tomography scan provides a more detailed picture of the extent of lung damage. Lung function and sputum tests can also be performed.
Diagnosis can also be made before a baby is born, using prenatal screening. This may be carried out via amniocentesis or chorionic villus sampling. For amniocentesis, a needle is inserted into the abdomen and passed through to the inside of the uterus where a small amount of fluid from the amniotic sac surrounding the fetus is removed. This amniotic fluid is tested to see if the CFTR genes are normal.
In the case of chorionic villus sampling, a thin tube is inserted through the vagina and cervix and into the uterus where a small sample of tissue is taken from the placenta. This sample is then tested for the CFTR mutation.
Genetic testing can be performed to check a person’s carrier status in those with a family member or partner who has the condition. For this test, cells from the inside of the mouth are taken and tested for CFTR gene mutations.
Reviewed by Sally Robertson, BSc