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Testing and Diagnosis of Cystic Fibrosis

Thanks to 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

Tests used to diagnose 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 CF through bloodwork (genetic testing). If the bloodwork shows that both parents are CFTR carriers, then amniocentesis is used to test the baby before birth.1-6

Tests used to confirm a diagnosis of CF may include some or all of the following:1

  • Sweat test: Measures the amount of salt in a person’s sweat
  • Genetic (DNA) testing: Looks for CFTR mutations

Borderline test 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 CF 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.8

In 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

Written by: Jessica Johns Pool | Last reviewed: September 2019
  1. Cystic Fibrosis Foundation. Diagnosed With Cystic Fibrosis. Available at: Accessed 5/16/2019.
  2. Saddi V, et Ooi CY, Jaffe A. Cystic Fibrosis. In: Kline MW, et al. Rudolf’s Pediatrics. 23rd edition. Vol 2. New York; NY; McGraw-Hill Education; 2018: 2444-2457.
  3. Wagener, J. S.; Zemanick, E. T.; Sontag, M. K. (2012). Newborn screening for cystic fibrosis. Current Opinion in Pediatrics. 24 (3): 329–335. doi:10.1097/MOP.0b013e328353489a. PMID 22491493.
  4. Johns Hopkins Cystic Fibrosis Center. Diagnosis: Testing: CFTR Mutation Analysis. Available at: Accessed 5/16/2019.
  5. Johns Hopkins Cystic Fibrosis Center. Diagnosis: Testing: Nasal Potential Difference. Available at: Accessed 5/16/2019.
  6. Johns Hopkins Cystic Fibrosis Center. Diagnosis: Testing: Other. Available at: Accessed 5/16/2019.
  7. Cystic Fibrosis Foundation. CFTR-Related Metabolic Syndrome (CRMS). Available at: Accessed 5/16/2019.
  8. Cystic Fibrosis Foundation. Patient Registry Annual Data Report 2017. Available at: Accessed 5/16/2019.
  9. Elborn JS. Cystic fibrosis. Lancet. 2016 Nov 19;388(10059):2519-2531. doi: 10.1016/S0140-6736(16)00576-6. Epub 2016 Apr 29.