Diagnosing cystic fibrosis (CF) is usually a multistep process that involves several evaluations and screening tests. This includes genetic and sweat chloride testing alongside clinical evaluation — where doctors look for signs and symptoms of CF.

CF is a genetic disease affecting the lungs, digestive system, and other organs. The body produces thick mucus, resulting in a chronic cough, among other symptoms.

Doctors may diagnose a person with CF based on their symptoms, such as wheezing and shortness of breath. They will also screen people for the condition with genetic and sweat chloride testing.

This article looks at the different tests for cystic fibrosis, including carrier testing, newborn screening, sweat chloride testing, and clinical evaluation.

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According to the Cystic Fibrosis Foundation, carrier testing, or genetic testing, plays a critical role in both the diagnosis of CF and family planning decisions.

The standard genetic test checks for the 23 most common gene mutations.

A child must inherit one copy of the cystic fibrosis transmembrane conductance regulator (CFTR) gene mutation from each parent to develop CF. People with only one copy of a CFTR gene mutation do not have CF, but experts refer to them as lCF carriers.

Blood, saliva, and cell samples from inside the cheek can help determine whether someone carries the faulty gene that causes CF.

What the results mean

If two CF carriers have a child, that child’s risk of CF is as follows:

  • There is a 25% chance they will have CF.
  • There is a 50% chance they will be a carrier of the gene but will not have CF.
  • There is a 25% chance they will not be a carrier and will not have CF.

People with CF may also pass copies of their CF gene mutations to their children. If someone with CF has a child with a CF carrier, then the child’s risk of CF is as follows:

  • There is a 50% chance they will be a carrier but will not have CF.
  • There is a 50% chance they will have CF.

Read more about genetics and CF.

In the United States, all newborns undergo screening for CF in the first few days of their lives. It is essential to diagnose CF early in babies to help prevent complications.

The test screens for the most common gene mutations, but it may also determine whether the infant is a CF carrier, even if they do not appear to have the condition. However, it is also possible for the test not to identify a rare genetic mutation. These individuals may not receive a CF diagnosis until later in life.

The screening involves taking a small blood sample or prick from the baby’s heel.

What the results mean

The Cystic Fibrosis Foundation suggests that tests check the levels of a chemical made by the pancreas called immunoreactive trypsinogen (IRT). IRT typically occurs at small levels in the body. However, for babies with CF, IRT levels are high. There are other reasons why it could be high — for example, a baby is premature.

Some U.S. states may test for IRT levels alongside DNA testing.

A positive result from the heel prick test suggests that a baby may have CF. However, further tests are necessary to confirm a diagnosis.

Find out more about CF in children.

According to the Cystic Fibrosis Foundation, medical professionals consider the sweat chloride test the most reliable diagnostic method, as people with CF tend to have more chloride in their sweat.

In the first part of the test, doctors apply a colorless, odorless chemical and a little electrical stimulation to a small area of the arm or leg. This encourages the sweat glands to produce sweat and may cause a tingling in the test area.

Doctors then collect sweat — using filter paper, gauze, or plastic coil — and send it to a laboratory for analysis.

Experts perform the sweat test on anyone older than 48 hours old. Doctors must repeat the test if an infant does not produce enough sweat the first time.

What the results mean

Doctors assess the amount of chloride in sweat. Generally, levels of chloride:

  • less than or equal to 29 millimoles per liter (mmol/L) mean CF is unlikely
  • between 30 and 59 mmol/L mean that CF is unclear, so people require additional testing
  • greater than or equal to 60 mmol/L mean CF is likely

If a child has a positive result on their newborn screening and a sweat test of 30–59 mmol/L, parents and caregivers should consult with a CF specialist to explain the results further. Other tests may be necessary.

Learn more about the sweat test.

Clinical evaluation means that doctors assess a person’s vitals and symptoms of the disease.

The Cystic Fibrosis Foundation notes that people with CF may have a variety of symptoms, including:

Research from 2017 notes other clinical signs doctors look out for, including:

Doctors diagnose CF using various diagnostic tools, including carrier testing, newborn screening, sweat chloride testing, and evaluation of symptoms.

Genetic testing for carriers checks the 23 most common gene mutations associated with CF. Blood, saliva, or cells from the cheek can be used to determine whether someone is a carrier of the faulty gene. Still, this same test cannot determine whether someone will have the condition. Carriers may not develop the condition.

Similarly, newborn screening assesses whether a baby is a carrier or has CF. It involves a prick of blood from the baby’s heel. A positive test may signal CF, but doctors recommend additional testing.

The sweat chloride test checks how much chloride is in a person’s sweat, as people with CF have high levels. Levels higher than 60 mmol/L in the sweat mean a person has CF. Clinical evaluation is also essential, as certain symptoms, likely salty-tasting skin, may cause doctors to suspect CF.