Cystic Fibrosis Diagnosis

Cystic fibrosis (CF) is a genetic disorder caused by a mutation in the CFTR gene. This leads to the buildup of thick, sticky mucus in various organs, including the lungs and intestines, causing inflammation and fibrosis (scarring) and leaving the patient vulnerable to recurrent infections.

The majority of CF cases are diagnosed early due to the implementation of newborn screening, which, as its name suggests, involves checking babies for CF (and usually other diseases simultaneously) shortly after the child is born. In some cases where the risk of disease is known to be high, prenatal testing — checking for CF while a fetus is still developing — may also aid with early diagnosis. That can enable earlier initiation of disease management, which is linked with better long-term outcomes.

Newborn screening has been implemented across all states in the U.S. and in many other countries, including the U.K. and Canada. Screening is typically carried out by taking a sample of blood, usually by pricking the baby’s heel; it may involve one or more specific diagnostic tests for CF.

Genetic testing

A blood or saliva sample can be taken for genetic testing, which checks for known CF-causing mutations in the CFTR gene. In the U.S. and U.K., this is carried out as part of newborn screening; it may also be useful in prenatal testing.

Genetic testing also may be carried out to confirm the type of mutation that is causing CF in a patient. This can be useful, as different known mutations are associated with different symptoms, and many treatments are targeted toward specific mutations.

Sweat test

Considered a gold standard for diagnosing CF, the sweat test involves measuring levels of chloride in a person’s sweat. Chloride is a salt — half of sodium chloride (table salt). In CF, chloride sweat levels are abnormally high due to dysfunction of the CFTR gene, which normally helps to regulate the movement of this salt in the body.

The test generally involves using a mild electrical current to stimulate sweat, which is then sent to a laboratory for analysis. Babies don’t usually produce enough sweat to do the test until they are at least 2 weeks old.

Immunoreactive trypsinogen (IRT) test

In CF patients, high levels of an enzyme precursor called immunoreactive trypsinogen, or IRT, accumulate due to the thick mucus that builds up in the ducts of the pancreas and blocks trypsinogen and other proteins from reaching the small intestine.

IRT testing is often done as part of newborn screening; however, in some cases, IRT levels may be high in newborns due to premature birth or a stressful delivery. Therefore, the IRT test must be accompanied by other tests for a definitive diagnosis of CF.

Nasal potential difference

The alterations in bodily salt movement that occur in CF cause abnormalities in conductance — essentially, small electrical charges. Nasal potential difference (NPD) is a test in which these differences in conductance can be measured by placing small electrodes in the nasal passages and measuring conductance using a voltmeter.

NPD is generally well-tolerated and has the advantage of providing results immediately, but the test itself takes a fair amount of expertise, and results may not be reliable in certain patients (for example, smokers or those with nasal polyps).

Other tests

A variety of other diagnostic tests may be useful in detecting CF and/or monitoring the disease’s progression. For example, imaging of the lungs using X-ray or CT scan may help detect physical damage to lung tissue. Analyses of fecal samples may be helpful for detecting pancreas dysfunction or abnormalities in gut bacteria.

 

Last updated: Nov. 15, 2021

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Cystic Fibrosis News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.