Tests CF Gene Mutation. Following recent recommendations from several major health organizations, a panel of 25 common mutations of the CF gene has been developed to screen general or targeted populations for CF and CF carrier status. (Some laboratories have larger panels of 30-88 mutations.) These panels check the patient’s
DNA for each of the selected CF mutations. If two CF mutations are identified, then the patient has CF; if one is located, the patient is either a carrier or has CF with the second mutation unidentified. Further clinical assessment, genetic testing and a
sweat test may be performed. If the patient tests negative for the 25 mutations and is asymptomatic, then the chances are that they do not have CF, but there is still a slight risk of them being a carrier of a rare mutation.
The Sweat Test. This test involves measuring sodium and chloride from a sweat sample collected by a special procedure in which local sweating is stimulated by pilocarpine, the sweat collected on weighed filter paper and the concentrations of sodium and chloride measured. Since the CFTR protein is altered or missing and chloride travel is restricted, the sweat in a CF person may be up to five times saltier than normal. Positives should be confirmed and followed with CF gene mutation testing. Some people with CF will be diagnosed using only sweat testing, and a few very rare patients with definite CF, often with unusual mutations, may have sweat sodium and chloride levels within the normal range.
Faecal chymotrypsin. Faecal chymotrypsin is a stool test for proteolytic enzymes, produced in an inactive form in the pancreas and then activated in the small intestine to digest food proteins. Low values indicate pancreatic insufficiency in untreated people with CF; in patients on pancreatic enzyme replacement therapy values are usually normal but do not correlate closely with the amount of fat in the stools.
Faecal Pancreatic Elastase 1. This is a specific human protease produced by the pancreas. Values determined on a small faecal specimen clearly differentiate between sufficient and insufficient pancreatic function and are not influenced by exogenous pancreatic enzyme treatment. This is now the best indirect measure of pancreatic function and it has superseded a number of indirect pancreatic function tests including the bentiromide and pancreolauryl tests.
Immunoreactive Trypsin (IRT). This newborn screening test for CF is measured using the blood spots collected on Guthrie screening cards. 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, and a raised second IRT usually combined with DNA testing is used in many countries for neonatal CF screening. Persistently raised blood IRT values after the newborn period usually indicate that the infant has CF. Early diagnosis is essential so that treatment can be started before damage to the lungs with chronic infection and
malnutrition become established.
Other laboratory tests used to check lung infection,
organ function and fertility include:
Non-laboratory tests that may be done include respiratory function tests, chest X-rays, lung scans, bronchoscopies, bone scans, upper GI and small bowel X-rays and other gastrointestinal pancreatic and liver investigations.
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