Question
How is cystic fibrosis diagnosed?
Answer
Sweat chloride testing is the gold standard for the diagnosis of Cystic Fibrosis (Figure 5). The sweat test needs to be done in a CF Foundation accredited center, where personnel is trained to obtain a sweat test. It provides a conclusive CF diagnosis by measuring the concentration of salt in a person's sweat. The simple, painless test is the most reliable way to diagnose Cystic Fibrosis. Infants do not sweat much. Sweat glands have to be stimulated by using an electrical current, along with a chemical called pilocarpine. We usually test on two sides. Often it is both forearms. After the stimulation, a plastic coil device the size of a quarter is placed and covered with gauze to collect the sweat. We advise families to keep babies hydrated and wrapped in a blanket to obtain enough sweat for the test.
After the sweat test is completed, how do you interpret the results? Well, 60 is the magic number. Always think 60 in Cystic Fibrosis. Sweats are measured in milliequivalent per liter units. Anything over 60 is a positive indication of Cystic Fibrosis. Sweat tests between 31 and 59 are intermediate and will need to be repeated. If the repeat sweat is also intermediate, we get a CFTR gene test, either with blood or a buccal swab. Any sweat tests below 30 are consistent with normal findings. As with any test, there is a chance for false positives and false negatives. Some conditions that can cause false positives include hypothyroidism, Addison's disease, and malnutrition. I have seen many malnourished patients with elevated sweats, and once their malnutrition is corrected, their sweat will normalize.
We can also get false negatives if the patient has edema, if the sample was diluted, or if they have milder CF mutations. There are patients with two Cystic Fibrosis mutations of milder severity that can have normal sweat tests. In which case, we follow them annually or every six months to see if they ever develop any signs and symptoms of Cystic Fibrosis.
This Ask the Expert is an edited excerpt from the course, Cystic Fibrosis: A Comprehensive Overview, presented by Prigi Varghese, MSN, APRN, CPNP-PC.
