Medical Edge Newspaper Column

Genetic Testing For Cystic Fibrosis Will Help With Diagnosis

February 11, 2008
Dear Mayo Clinic:
My 5-year-old daughter has been tested for cystic fibrosis twice. Both times the test has come up in the "gray area" in the 40s. The pulmonologist told us he does not know a lot about the gray area. My daughter is growing normally but continues to have very thick mucus. She has been tested for allergies and has none. She does have some digestive problems — stomach pain and occasional foul-smelling, mucous stools. What is your take on this?

Answer:
Cystic fibrosis affects the glands in your body that produce sweat, mucus, saliva and digestive juices. Usually, these secretions are thin and slippery, acting as lubricants throughout your body. Cystic fibrosis causes lung mucus and digestive tube mucus to become thick and sticky, so they clog the body's breathing tubes and digestive tract.

Lung problems, the most feared complication in cystic fibrosis, stem from thick airway mucus that promotes lung infection, eventually leading to lung destruction. The disease can also result in digestive problems and, in some cases, malnutrition. This is because thick secretions prevent your body from absorbing vitamins through the lining of the intestines. They also keep enzymes produced by the pancreas from reaching the intestine, which affects proper digestion of fats and proteins. But not everyone who has cystic fibrosis experiences all these problems. Signs and symptoms can vary significantly from one person to another, depending on the severity of the disease.

A diagnosis of cystic fibrosis is usually based on the results of a sweat test, in combination with the existence of signs and symptoms of the disease. A sweat test involves collecting sweat from two sites on a person's forearm and then measuring and comparing the amount of chloride in the samples. There's salt in everyone's sweat, but the amount of chloride (and sodium) in people who have cystic fibrosis is typically two to four times greater than normal. Unfortunately, as you've discovered, there is a gray zone between normal sweat chloride (less than 30 milliequivalents per liter) and that of most people with cystic fibrosis (typically more than 60 milliequivalents per liter). That gray zone is 40 to 60 milliequivalents per liter.

Because your daughter has thick mucus — a possible symptom of cystic fibrosis — in addition to chloride levels in the gray area, I'd recommend she undergo a genetic blood test to help determine if she has cystic fibrosis. Cystic fibrosis is an inherited condition, and genetic tests help us detect gene mutations related to the disease. The test is 85 percent accurate and especially useful in gray zone cases. (It also can be used to screen family members to see if they are carriers of the condition.) In addition to genetic testing, clinical tests to analyze her lung and digestive function may be helpful.

Not all forms of cystic fibrosis are the same. Many people with sweat test results in the gray area have no disease, an atypical form of cystic fibrosis or a different medical condition altogether. Some adrenal, thyroid and pituitary gland disorders, as well as some pancreatic conditions, can also cause sodium and chloride levels to be in the gray area. Talk to your daughter's physician about additional evaluation, so she can receive a definitive diagnosis and get appropriate treatment.

— Mark Wylam, M.D., Pulmonary and Critical Care Medicine, Mayo Clinic, Rochester, Minn.