Cystic fibrosis (CF) is a genetic disorder caused by a mutation in the CFTR gene. This leads to the build-up of thick, sticky mucus in various organs, including the lungs and intestines, causing inflammation and scarring (fibrosis) and leaving the patient vulnerable to recurrent infections. For a full diagnosis of CF, multiple tests need to be carried out.
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 a baby is born. An early diagnosis can prevent or delay many serious problems and improve growth, quality of life, and life expectancy.
Newborn screening involves placing a few drops of the baby’s blood, usually taken from the heel, 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 must be accompanied by other tests for a definitive diagnosis of CF.
A blood or saliva sample can be taken for genetic testing. This will be used to check for known mutations in the CFTR gene. In some U.S. states and in the U.K., this is carried out as part of newborn screening.
Prenatal screening may be offered if the parents are known or suspected to be carriers of a CFTR mutation. Using a technique called amniocentesis, a sample of the amniotic fluid (the liquid surrounding the developing baby, which contains cells shed from the fetus) is taken and a DNA test is carried out. Alternatively, chorionic villus sampling may be used to collect a small piece of placenta for genetic testing. The procedure can be performed transabdominally (through the abdomen) or transcervically (through the cervix). The placenta contains genetic material from the fetus and can be used to test for CF.
Genetic testing also may be carried out to confirm the type of mutation that is causing CF in a patient. This can be useful, as different known mutations are associated with different symptoms, and many treatments are targeted toward a specific mutation.
The sweat test is usually carried done 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, a 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. A high concentration of chlorine indicates a positive result for CF.
Nasal potential difference
CF causes differences in skin conductance because of the alterations in salt transport. Nasal potential difference (NPD) is a test in which these differences in conductance can be measured by placing small electrodes in the nasal passages and measuring conductance using a device called a voltmeter.
In older children and adults who were not previously diagnosed with CF, but show the symptoms of the condition, other tests may be done, including chest and sinus CT scans, lung function tests, tests for pancreatic enzymes in stool samples (fecal elastase), and tests for bacteria associated with CF.
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