About Cystic Fibrosis

Nevada Newborn Screening Program

Newborn Screening For Cystic Fibrosis

Cystic fibrosis is a progressive, genetic disease that causes persistent lung infections and limits the ability to breathe over time. In people with CF, a defective gene causes a thick, buildup of mucus in the lungs, pancreas and other organs. In the lungs, the mucus clogs the airways and traps bacteria leading to infections, extensive lung damage and eventually, respiratory failure. In the pancreas, the mucus prevents the release of digestive enzymes that allow the body to break down food and absorb vital nutrients.
Cystic fibrosis is a complex disease and the types and severity of symptoms can differ widely from person to person. Many different factors, such as age of diagnosis, can affect an individual's health and the course of the disease.

Frequently Asked Questions

What are the Symptoms of CF?

People with CF can have a variety of symptoms, including:

  • Very salty-tasting skin
  • Persistent coughing, at times with phlegm
  • Frequent lung infections including pneumonia or bronchitis
  • Wheezing or shortness of breath
  • Poor growth or weight gain in spite of a good appetite
  • Frequent greasy, bulky stools or difficulty with bowel movements
  • Male infertility
How Does Someone Get CF?

Cystic fibrosis is a genetic disease. People with CF have inherited two copies of the defective CF gene—one copy from each parent. Both parents must have at least one copy of the defective gene.

People with only one copy of the defective CF gene are called carriers, but they do not have the disease. Each time two CF carriers have a child, the chances are:

  • 25 percent (1 in 4) the child will have CF
  • 50 percent (1 in 2) the child will be a carrier but will not have CF
  • 25 percent (1 in 4) the child will not be a carrier and will not have CF

The defective CF gene contains a slight abnormality called a mutation. There are more than 1,700 known mutations of the disease. Most genetic tests only screen for the most common CF mutations. Therefore, the test results may indicate a person who is a carrier of the CF gene is not a carrier.

More than 10 million Americans are carriers of a faulty CF gene. Many of them do not know that they are CF carriers.

Who is Affected by CF?

According to the Cystic Fibrosis Foundation Patient Registry, in the United States:

  • More than 30,000 people are living with cystic fibrosis (more than 70,000 worldwide)
  • Approximately 1,000 new cases of CF are diagnosed each year
  • More than 75 percent of people with CF are diagnosed by age two
  • More than half of the CF population is age 18 or older

Cystic fibrosis (CF) affects both males and females and people from all racial and ethnic groups. However, the disease is most common among Caucasians of Northern European descent. CF also is common among Latinos and American Indians, especially the Pueblo and Zuni. The disease is less common among African Americans and Asian Americans.

What is a Sweat Test?

The sweat test is considered the gold standard for diagnosing CF. Sweat tests should be done at a CF Foundation-accredited care center, where strict guidelines help ensure accurate results. The sweat test is performed by a trained technician and the results are evaluated in an experienced and reliable laboratory.

The sweat test can be done on an individual of any age. However, some infants may not make enough sweat to do the test. If an infant does not produce enough sweat the first time, the test should be repeated.

What to Expect During a Sweat Test

The sweat test measures the amount of chloride in the sweat. There are no needles involved in this test. In the first part of the test, a colorless, odorless chemical (pilocarpine) and a little electrical stimulation is applied to a small area of an arm or leg to encourage the sweat glands to produce sweat. A person may feel tingling in the area, or a feeling of warmth. This part of the test lasts about five minutes.

The sweat is then collected on a piece of filter paper or gauze or in a plastic coil. This step lasts for 30 minutes. The collected sweat is sent to a hospital laboratory to measure how much chloride is in the sweat. The sweat test usually takes about an hour, but it may take longer. When you schedule the test, ask how long it will take and when you can expect to learn the results.

Understanding Sweat Test Results

People with CF have more chloride (a component of salt) in their sweat than people who do not have CF. For a child who has CF, the sweat chloride test results will be positive (showing a high chloride level) shortly after birth. A baby has to sweat enough to do the test. Full-term babies usually produce enough sweat by 2 weeks of age. The test is typically done between two and four weeks of age for babies who have had a positive newborn screen.

Sweat chloride values do not change from positive to negative or negative to positive, as a person grows older. Sweat test results also do not vary when a person has a cold or other brief illness. If a sweat test is done correctly, then results that are positive will show a high chloride level.

For infants up to and including 6 months of age, a chloride level of:

  • Equal to or less than 29 mmol/L (negative) = CF is very unlikely
  • Between 30 - 59 mmol/L (intermediate) = CF is possible
  • Greater than or equal to 60 mmol/L (positive) = CF is likely to be diagnosed

For people older than 6 months of age, a chloride level of:

  • Equal to or less than 39 mmol/L (negative) = CF is very unlikely
  • Between 40 - 59 mmol/L (intermediate) = CF is possible
  • Greater than or equal to 60 mmol/L (positive) = CF is likely to be diagnosed

In a small number of cases, sweat chloride test results fall into an uncertain or borderline range. These results are described as intermediate and fall between not having CF and having CF. When this happens, the sweat test is usually repeated. In addition, other tests such as genetic testing may be done after you talk with your doctor.

Preparing For a Sweat Test

There is no activity limit or special diet needed before the sweat test. However, you should not apply creams or lotions to the skin 24 hours before the test. You can continue on all regular medications. These will have no effect on the test results. Babies should be fed their usual amount at their usual times.

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