Testing and Diagnosis of Cystic Fibrosis

Reviewed by: HU Medical Review Board | Last reviewed: September 2019

Thanks to improving testing types such as newborn screening, most people with CF are now diagnosed with cystic fibrosis soon after birth. However, people who were born before 2010 (the year the last U.S. state implemented newborn screening for CF) and those with a rare version of the disease may not be diagnosed until they begin showing symptoms.1-3

More than 1,700 variations of the CF genetic mutation have been identified so far. The most common variations of the CF genetic mutation cause symptoms in childhood. The rarer kinds usually are not identified through newborn screening and may not cause any symptoms until adulthood.1

Testing for cystic fibrosis

A few babies are diagnosed during pregnancy through ultrasound prenatal tests. If an ultrasound reveals problems that point to CF (usually bowel obstruction), the parents are tested for cystic fibrosis through bloodwork (genetic testing). If the bloodwork shows that both parents are CFTR carriers, then amniocentesis is used to test the baby before birth.1-6Tests used to confirm a diagnosis of CF may include some or all of the following:1

Borderline testing results

If newborn screening suggests a diagnosis of CF, a sweat test and DNA test will be performed. In some children, these tests may have what are called inconclusive or borderline results. This is known as CFTR-Related Metabolic Syndrome (CRMS) in the U.S. and Cystic Fibrosis Screen Positive, Inconclusive Diagnosis (CFSPID) in Europe.2-4

These children may not show any symptoms of CF until adulthood because they have some partial function of the CFTR protein. People with borderline test results should be monitored closely and see cystic fibrosis specialists because they have a higher risk of problems with their lungs, sinuses, pancreas, intestines, and reproductive system.6,7 Around 10% of newborn screenings give an inconclusive result.8In difficult to diagnose cases, doctors may use these additional tests:

  • Nasal potential difference test: Measures salt transport in the skin inside the nose
  • Fecal elastase: Measures how well the pancreas is working1-6

Adult-onset cystic fibrosis

People who are not diagnosed with cystic fibrosis until adulthood often have been misdiagnosed with allergies, asthma, or other respiratory conditions earlier in life. Adult-onset CF is not a mild form of CF. Once properly diagnosed, these people experience a decline in lung and often pancreatic function similar to those identified with CF in childhood.9

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