Cystic Fibrosis

Tests

CF Gene Mutation. The American College of Medical Genetics and the American College of Obstetricians and Gynecologists recommend that a panel of 23 of the most common CFTR mutations be used to screen general or targeted populations for CF and CF carrier status. (Some laboratories have larger panels of 30-96 mutations.) These panels check the DNA of each member of a couple contemplating a pregnancy or during early pregnancy for each of the selected CF-causing mutations. If two CF mutations are identified, one mutation in each of the parents, then a genetic counselor should be consulted to explain the relative risk the couple has of conceiving a child that may have CF.

Sweat Chloride. This test involves measuring chloride from a sweat sample collected through a special procedure. One of the classic symptoms of CF is extraordinarily salty sweat. Since the CFTR protein is altered or missing and chloride travel is restricted, the sweat of a person with CF may be up to five times saltier than normal. Positive sweat chloride test results should be confirmed and followed with CF gene mutation testing wherever possible. Some cases of CF will be diagnosed using only sweat chloride testing, and a few rare cases of CF may have normal sweat chloride levels.

Trypsin/chymotrypsin. This is a stool test for proteolytic enzymes, which are produced in an inactive form in the pancreas and then activated in the small intestine to digest dietary proteins. A positive test detecting the presence of these enzymes is normal. This is a screening test for pancreatic sufficiency.

Immunoreactive Trypsinogen (IRT). This test is a newborn screening tool for trypsinogen, which is produced in the pancreas and transported to the intestine, where it is activated to form the enzyme trypsin. In CF, thick mucus plugs can obstruct pancreatic ducts and prevent trypsinogen from reaching the intestine. Blood IRT levels will be elevated in newborns with CF, but positive results must be followed by confirmatory testing as there is a high rate of false positive test results (positive results due to conditions other than CF and/or temporary elevations).

Nasal (transepithelial) Potential Difference (NPD). The active transport of ions, primarily sodium and chloride, across the respiratory epithelium, a layer of cells that line the nasal cavity, generates a potential difference (PD) that can be measured. The abnormal sodium and chloride transport in the respiratory epithelia of patients with CF is associated with a different pattern of nasal PD compared with normal epithelia. The NPD technique examines particular features of this NPD to help establish a diagnosis of CF.

Other tests used to check organ function, fertility, and to detect lung infections include:

Non-laboratory tests that may be done include bone and chest X-rays, upper GI and small bowel series, and lung function tests.

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