Cystic fibrosis (CF) is an inherited disorder that causes thick mucus to build up in the lungs, digestive system (and pancreas) and other organs. Most people with CF get frequent chest infections. They may also have problems digesting their food and, as a result, may not gain weight as well as they should. Newborn screening means that babies with CF can receive early treatment with medicines and physical therapy that help them digest food and keep their lungs clear of mucus. This treatment will help prevent serious illness and allow the child to live a healthier life. About one baby in 4,300 in BC is born with CF.

For more information:

• Cystic Fibrosis Care BC (phsa.ca)
  
• Cystic Fibrosis Information Sheet
  
• Cystic Fibrosis Canada 
  

Screening

The process used to screen for cystic fibrosis is described below:

1Measure IRT in Blood Spot Card

Using the blood spots from a baby’s heel prick, the level of a protein in your baby’s blood is measured. The protein is called immunoreactive trypsinogen (IRT). In babies with CF the IRT level is higher than normal.

• If the IRT level is normal, this is considered a negative screen for CF. No further testing is required.
  
• If the IRT level is higher than normal, the lab will do another test on the same blood sample. This test is called a DNA test (see step 2).

The DNA test looks for common changes in the CF gene. What is the CF gene? A gene is a set of instructions like a recipe on how to make a protein.  When the instructions are changed the protein is does not work properly or is not made. A normally working CF protein is needed to stop the mucus in our body from becoming thick. The DNA test looks for common changes (called pathogenic variants) in the CF gene. We all have two copies of the CF gene.  

• Babies with a change (pathogenic variant) in both copies of the CF gene are likely to have CF.
  
• Babies with a change (pathogenic variant) in one copy of the CF gene are likely healthy "carriers" for CF. A carrier has one working copy of the CF gene and one non-working copy. 
• Babies with no changes (no pathogenic variants) in the CF gene are very unlikely to have CF and the newborn screen is reported to your health care provider as a negative screen.
  

The next step depends on the results of the DNA test. 

3Repeat IRT Test

If the baby has a change (pathogenic variant) in one copy of the CF gene, a repeat blood sample is taken when the baby is 21 days old. This sample is tested to see if your baby's IRT level is still increased. Most of the time the repeat IRT is normal.

• If the baby's second IRT level is normal then it is very unlikely that the baby has CF. The baby is much more likely to be a healthy CF carrier. If the parents want added reassurance they can request a sweat test on their baby.
  
• If the baby's second IRT level is still increased, the baby may have CF or may be a healthy CF carrier. A sweat test will be done to learn more.
  

• Information Sheet: High IRT and One Disease-Causing Variant
  
• Information Sheet: High IRT and Two Disease-Causing Variants 
  

4Sweat Test

A sweat test is arranged if the repeat IRT is elevated or if a baby has a change (pathogenic variant) in both copies of the CF gene. A sweat test will be done to find out if the baby does or does not have CF.