Diagnosis of Cystic Fibrosis
One of the tests used to diagnose CF is a computerized tomography (CT) scan. It uses a series of X-rays from multiple angles to build a 3-D image of bones, blood vessels, and soft tissues within the body. Chest CT scans can reveal both mucus and bronchiectasis (a thickening of the walls of the small compartments of the lungs called the bronchi) that may indicate infection, inflammation, and potential lung damage.
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 CF, genetic testing provides information about whether a person carries a mutation in the CFTR gene. Genetic testing is done by taking genetic material from cells, which are collected from a blood, saliva, or tissue sample from inside the cheek.
Immunoreactive trypsinogen (IRT) is an enzyme precursor that is normally made in the pancreas. In CF, IRT can be secreted at higher levels into the bloodstream because of blockages in the pancreatic ducts. IRT levels can also be high following a premature or difficult birth, so a positive test result must be confirmed by additional testing to make a correct CF diagnosis. IRT tests are part of standard newborn blood screens in the U.S. and U.K.
Nasal Potential Difference
The transport of ions such as sodium and chloride creates voltage or electrical potential difference across the airway lining. This potential difference can be measured through the nose (known as the nasal potential difference) by placing an electrode on the lining of the nose. This is used as an important diagnostic tool for CF and can be used to assess the effectiveness of different treatments.
In the U.S., Canada, Australia, and in most of Europe, newborn infants are tested for a variety of health conditions before leaving the hospital. One of the diseases tested for is CF. In the first 24 to 48 hours after birth, a few drops of blood from the infant is collected with a heel prick. The blood is placed on a card and is analyzed to determine how much immunoreactive trypsinogen (IRT) — a protein commonly elevated in cystic fibrosis — is present.
A prenatal diagnostic test can help identify, before birth, whether a fetus is carrying the CFTR mutation that causes CF. Test results have a high degree of certainty about the CF status of the fetus, but cannot predict disease severity. Two types of prenatal diagnostic tests can be performed depending on the stage of the pregnancy: chorionic villus sampling or amniocentesis.
Patients with CF have more chloride in their sweat due to the lack of functional CFTR protein. The sweat test is used to confirm a diagnosis after a positive newborn screening for CF. The test is also frequently used in clinical trials as a measure to evaluate whether a particular treatment improves CFTR function. It’s considered by clinicians to be the gold standard in diagnosing the disease.