Diagnosis: Cystic fibrosis
• Trypsinogen is produced in the pancreas and transported to the intestine, where it is activated to form the enzyme trypsin. In cystic fibrosis, thick mucus can obstruct the pancreatic ducts and prevent trypsinogen from reaching the intestine. Therefore, blood levels of immunoreactive trypsinogen will usually be elevated in newborns with cystic fibrosis.
• There are a considerable number of false-positive immunoreactive trypsinogen results. A positive immunoreactive trypsinogen result triggers the performance of a DNA mutation analysis.
• A negative immunoreactive trypsinogen result in a patient with cystic fibrosis, although rare, can be obtained and does not routinely trigger a DNA mutation analysis. If cystic fibrosis is suspected, the patient should undergo genetic testing or have a sweat chloride measurement.
• In the nasal potential difference method, the active transport of ions, mainly sodium and chloride, across the respiratory epithelium generates a potential difference that can be measured. The abnormal sodium and chloride transport in the respiratory epithelia of patients with cystic fibrosis can be measured and used to diagnose patients with this disease.
• Sweat conductivity and sweat osmolality are screening tests for cystic fibrosis. A positive result must be confirmed by sweat chloride measurements.