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How to diagnose cystic fibrosis

In the UK, all newborn babies are tested for cystic fibrosis.

Blood spot test

The blood spot test is sometimes called the heel prick test. A small sample of blood is taken from your baby’s heel. You’ll be offered this when your baby is 5 days old. This blood is sent to a lab where it’s tested for a number of conditions, including cystic fibrosis.

If the blood spot test shows there may be a problem, your baby will need some more tests to confirm the diagnosis.

Sweat test

A sweat test involves taking a sample of your child’s sweat to test how much salt it contains. People with cystic fibrosis have more salt in their sweat. This is the best way to diagnose cystic fibrosis.

Genetic test

Your child might also have a genetic test. A doctor will take a blood sample. This will be tested for the genes that cause cystic fibrosis.

Testing family members

If your child is diagnosed with cystic fibrosis, any brothers or sisters they have will also need to be tested for the condition.

If a member of your family is diagnosed with cystic fibrosis, there’s a chance that you could be a carrier of the faulty gene. It’s a good idea to have a carrier test to find out if you are a carrier. Your doctor or nurse will take a genetic sample using a special mouthwash or take a blood sample to be tested.

Next: How is cystic fibrosis treated? >

Last medically reviewed: September 2019. Due for review: September 2022

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.

Cystic Fibrosis (CF)

What Are the Symptoms of Cystic Fibrosis?

Symptoms of lung disease can start in infancy, especially following upper respiratory viral infections. People with CF experience a small but progressive (worsening) loss in lung function with every passing year, leading to increased symptoms as you age. Some children remain relatively healthy throughout childhood and only start to experience a decline in their lung function when they are teenagers.

There is a wide range of severity in CF symptoms. Even within the same family, siblings can have different disease severity. Symptoms of CF can be classified into two main categories: respiratory and digestive.

The most common symptoms of CF respiratory tract disease are:

  • Chronic coughing (dry or coughing up mucus)
  • Recurring chest colds
  • Wheezing or shortness of breath
  • Frequent sinus infections
  • Very salty-tasting skin

Digestive symptoms may include greasy, foul-smelling bowel movements, severe constipation or intestinal blockage and the inability to gain weight while being constantly hungry.

How Cystic Fibrosis Is Diagnosed

  • Newborn screening. In the last decade, newborn screening has become standard and is now available in all 50 U.S. states. The newborn screen shows infants who have a high level of an enzyme called immunoreactive trypsin in their blood. This occurs when there is injury to the pancreas. The test is repeated if it is abnormal. Some states also combine this with testing for the most common gene mutation called deltaF508. The next step is to refer the infant for further testing as there are many “false-positive” tests. This entails taking a blood sample to check whether the infant has two genes that cause CF and/or performing a sweat test.
  • Genetic testing. More than 2,000 different mutations of the CF gene have been identified. Most of them are quite rare, but a few are common, like the deltaF508 mutation that is found in at least 70% of individuals with CF. Genetic testing can determine the exact mutation in most cases. For couples who want to have children, genetic testing is also important as more than 10 million Americans are carriers of a CF gene. For every pregnancy, there is a one-in-four chance that the child will have CF when both parents are carriers.
  • Sweat test. Sweat is collected from a small area on the child’s forearm, and the chloride levels are measured. Children with CF have high levels of chloride in their sweat because a lack of CFTR prevents the salt on the skin from being reabsorbed back into the sweat glands.
  • Measuring nasal lining. Another way to confirm the diagnosis is to run a small electrical current across the nasal lining (epithelium). Different solutions are applied to the nasal lining and the electrical current is measured. People with CF respond very differently than those without CF to this test, and it may help confirm a diagnosis.

When to See Your Doctor

If you or a loved one have a family history of CF and have pulmonary symptoms, have been diagnosed with CF, or experience these symptoms, consult your healthcare provider and request an evaluation at an accredited CF center. The Cystic Fibrosis Foundation website has a tool to assist in locating a care center.

If Cystic Fibrosis is suspected for any reason, patients are referred to Stanford for testing. A complete diagnostic workup can be done at the clinic. Diagnostic tools include a sweat chloride test and a Cystic Fibrosis DNA mutation test.

In an effort to provide more than the commercial testing for the 70 most common mutations, our Center has published a method on complete genotyping. We also provide testing for Nasal Potential Difference on a research basis. Ours is one of only a few Centers in the country to offer this relatively new way helping to diagnose Cystic Fibrosis.

Sweat chloride, a non-invasive test, can aid in the diagnosis of Cystic Fibrosis because CF causes increased levels of sodium and chloride in the sweat. The test measures the level of chloride found in the sweat. No special preparation is necessary. The test is painless. An abnormal result may indicate the presence of Cystic Fibrosis.

CF DNA mutation analysis is a genetic test using either blood or a swab from the inner cheek. The test is used to diagnosis CF and also in research that is being done to search for a cure.

Everyone inherits two copies of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. However, some of the inherited copies are mutations (genetic material that is permanently changed in some way). Inheriting two mutated copies of this gene means that the person has Cystic Fibrosis. Inheriting only one copy of a mutated gene means that the person is a carrier for Cystic Fibrosis and could pass on a risk for CF to his or her childen.