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Cystic Fibrosis: Tests


The availability of reliable testing is one of the most significant advances in our understanding and treatment of CF. Testing is important not only for children and adults suspected of having the disease, but for individuals without symptoms that are concerned about the risk of being a CF carrier, with the potential to pass the gene to their children. Several different tests are currently available.

 

Newborn Screening

While long available, newborn screening for cystic fibrosis is now required in 36 States. Many studies have proven that CF diagnosed early in life results in healthier children, and even increased survival, when compared to children with a delayed diagnosis [3]. Thus, newborn screening is recommended by the United States Centers for Disease Control (CDC) and the CF Foundation. The test detects elevated quantities of trypsinogen in a drop of blood collected from the heel. This procedure is already widely performed to test for other congenital disorders. Newborn screening is not a definitive diagnostic test for cystic fibrosis, but, if positive, indicates the need for additional testing to rule out or confirm a CF diagnosis.

 

Sweat Test

The sweat chloride test is the classic method to diagnose CF in children and adults suspected of having the disease. The test is based on the principal that the sweat of CF patients is dramatically more salty than the general population; thus, a high chloride level indicates CF. While the test is painless, the procedure is somewhat complicated, and reliable results depend on very careful administration of the test. Sweat tests must be performed by trained technicians and evaluated in an experienced, reliable laboratory, or at a Cystic Fibrosis Foundation-accredited Care Center where strict guidelines help ensure accurate results. Sweat chloride concentrations less than 40 mmol/L are normal, but sweat chloride concentrations greater than 60 mmol/L are consistent with the diagnosis of CF. All positive results need to be confirmed with a repeat test on a separate day. Detailed information about the sweat test for patients and families is available from the United States CF Foundation.

 

Genetic Testing

Several tests are available that can identify the presence of defective CFTR genes in newborns with positive screening results, as well as children and adults with symptoms of CF. In addition, these tests are useful in identifying symptomless carriers of an abnormal CF gene, including an estimated 10-12 million Americans. The American College of Obstetricians and Gynecologists now recommend that all pregnant women and their partners, as well as couples planning to have children, have genetic testing to screen for carriers of CF [4]. Even CF patients confirmed to have the disease by the sweat test can benefit from genetic testing, as identifying the specific CFTR mutations can help predict the future severity of the condition. In addition, identification of which mutations each patient has may someday have therapeutic importance, as experimental therapies are in clinical trials to improve defective CFTR function for specific classes of mutations.

Genetic testing analyzes a person’s DNA (genetic material), which can be obtained from a blood sample or from cells that are gently scraped from inside the mouth. Currently, most genetic testing looks for the presence of between 30 and 90 of the most common CFTR mutations, which accounts for up to 90% of all cases in specific ethnic groups. However, as there are approximately 1500 known CFTR mutations, these screening panels have the potential to miss rare mutations that often produce more mild forms of the disease. In cases where CF is strongly suspected, despite the presence of only one mutation from previous testing, the entire CFTR gene can be sequenced to definitively determine the presence or absence of two disease-causing mutations.

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