Diagnosing Cystic Fibrosis

Cystic fibrosis is an inherited condition that prevents the body from reabsorbing sodium chloride, a type of salt found in sweat. In people with this condition, mucus that normally protects the lining of the respiratory and digestive systems does not have enough salt and water, making it thick and sticky. Bacteria often flourish in the mucus that builds up in the airways and can develop into chronic and severe lung infections.

Mucus also collects in the pancreas, which can cause inflammation and prevent the pancreas from producing the digestive enzymes needed to absorb certain nutrients and fats. In children with cystic fibrosis, this lack of pancreatic enzymes can cause digestive problems, difficulty gaining weight, and slower growth. Inflammation can also damage the cells that make insulin, a hormone made in the pancreas that helps the body metabolize sugar, increasing the risk of developing diabetes.

Although cystic fibrosis is a life-threatening condition, medical advances have enabled people with this condition to enjoy a significantly better quality of life than in the past. Thanks in part to mandatory cystic fibrosis screening in newborns, most people are now diagnosed with the condition during infancy and can begin treatment as soon as possible, which research shows leads to a healthier and longer life.

Depending on the particular type of genetic mutation causing a person’s cystic fibrosis, the condition may still go undetected during infancy, until symptoms such as repeated respiratory infections or unresolved digestive problems appear later in childhood or in early adulthood. To accurately diagnose this condition, our team of experts evaluates all available information—including the results of your child’s screening tests, genetic tests, and information about your family’s medical history—before performing a diagnostic assessment called a sweat test. 

Sweat Test

Each year, NYU Langone performs more than 200 sweat tests. This test, which measures the concentration of salt in sweat, is the gold standard for diagnosing cystic fibrosis. A high level of sweat chloride confirms a diagnosis of cystic fibrosis.

During the test, the laboratory technician places a gel called pilocarpine on a small area of your child’s arm or leg to cause sweating. The technician then puts a small electrode on the skin at that spot, creating a weak electrical current to further stimulate sweating. The electrode may cause tingling or warmth but is not painful. The sample of your child’s sweat is collected in a coiled tube for about 30 minutes and is then taken to a laboratory for analysis. You can stay with your child during this procedure. The results are typically available the next day.

A sweat chloride level lower than 30 means that your child does not have cystic fibrosis. A chloride level greater than 60 confirms a diagnosis. A borderline sweat test result—when the chloride level is between 30 and 60—means your child may have a metabolic syndrome related to an abnormal cystic fibrosis transmembrane conductance, or CFTR, gene. Children with this condition do not have strong evidence of cystic fibrosis in their sweat but do have this abnormal gene, which can increase the risk of having cystic fibrosis symptoms in the future. 

Children with a borderline sweat test can have additional genetic testing to look for the presence of a second CFTR mutation. Doctors at NYU Langone’s Cystic Fibrosis Center monitor infants and children with CFTR-related metabolic syndrome for symptoms of cystic fibrosis at least once a year.

Genetic Testing

If the sweat test results are inconclusive because the amount of salt in your child’s sweat is only slightly higher than normal, the doctor may recommend a blood test to confirm a diagnosis and to look for any of the more than 1,800 genetic mutations in the CFTR gene that can increase the risk of cystic fibrosis. This information can help our doctors to predict whether your child is likely to be symptom-free or has a higher risk of developing symptoms of cystic fibrosis.

More Cystic Fibrosis Resources