Cystic fibrosis (CF) is a genetic disorder caused by a mutation in the CFTR gene. This causes a thick mucus to build up in various organs, including the lungs and intestines, leading to infections, inflammation, and scarring (fibrosis). For a full diagnosis of CF, multiple tests need to be carried out.
Nowadays, the majority of CF cases are diagnosed early, due to the implementation of newborn screening across all states in the U.S and in many other countries, including the U.K. and Canada. Screening is often carried out two or three days after birth. An early diagnosis and treatment can prevent or delay many serious problems and can improve growth, quality of life, and life expectancy.
Newborn screening involves placing a few drops of the baby’s blood onto a special card, called a Guthrie card, to carry out a number of tests.
Tests for CF
Immunoreactive trypsinogen (IRT) test
In CF patients, high levels of a chemical called immunoreactive trypsinogen, or IRT, is released from the pancreas. IRT is normally very low or undetectable in the blood of healthy individuals. However, in some cases, IRT levels may be high in newborns due to premature birth or a stressful delivery. Therefore the IRT test will be accompanied by other tests for a definitive diagnosis of CF.
A blood or saliva sample can be taken for a DNA test. This will check for known mutations in the CFTR gene that cause CF. In some U.S. states and in the U.K., this is carried out as part of the newborn screening.
Prenatal screening may be offered if the parents are known to be carriers of a CFTR mutation. Using a technique called amniocentesis, a sample of the amniotic fluid (the liquid surrounding the developing baby) is taken and a DNA test is carried out.
Genetic testing may also be carried out to confirm what mutation is the cause of CF in a patient. This can be useful, as different known mutations are associated with different symptoms.
The sweat test is commonly carried out to confirm or rule out a diagnosis of CF. CF patients have very salty sweat due to high levels of chloride (a component of salt) and, unlike the IRT test, the chlorine levels are not normally affected by other factors.
In general, the baby will be at least two weeks old before enough sweat can be produced for the test to be viable. During the test, sweat production is stimulated with a minor electrical pulse. The chlorine levels in the collected sweat are measured in a laboratory, with a high concentration of chlorine indicating a positive test for CF.
In older children and adults who were not previously diagnosed with CF, but show the symptoms of the condition, other tests may also be done, including chest and sinus x rays, lung function tests, tests for pancreatic enzymes in stool samples (fecal elastase), and tests for bacteria associated with CF.
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