Cystic Fibrosis Testing
Parents talk about the sweat test, a newborn screening for cystic fibrosis. Watch Video »
Recent high-tech medical advances in drug therapy and genetics are helping children born with CF lead longer and more comfortable lives. In the last 10 years, research into all aspects of CF has helped doctors to understand the illness better and to develop new therapies. Ongoing research may someday lead to a cure.
CF affects more than 30,000 kids and young adults in the United States. It disrupts the normal function of epithelial cells — cells that make up the sweat glands in the skin and that also line passageways inside the lungs, liver, pancreas, and digestive and reproductive systems.
The inherited CF gene directs the body's epithelial cells to produce a defective form of a protein called CFTR (or cystic fibrosis transmembrane conductance regulator) found in cells that line the lungs, digestive tract, sweat glands, and genitourinary system.
When the CFTR protein is defective, epithelial cells can't regulate the way that chloride (part of the salt called sodium chloride) passes across cell membranes. This disrupts the essential balance of salt and water needed to maintain a normal thin coating of fluid and mucus inside the lungs, pancreas, and passageways in other organs. The mucus becomes thick, sticky, and hard to move.
Normally, mucus in the lungs traps germs, which are then cleared out of the lungs. But in CF, the thick, sticky mucus and the germs it has trapped remain in the lungs, which become infected.
In the pancreas, thick mucus blocks the channels that would normally carry important enzymes to the intestines to digest foods. When this happens, the body can't process or absorb nutrients properly, especially fats. Kids with CF have problems gaining weight, even with a normal diet and a good appetite.
Humans have 23 pairs of chromosomes made of the inherited genetic chemical deoxyribonucleic acid (DNA). The CF gene is found on chromosome number 7. It takes two copies of a CF gene — one inherited from each parent — for a child to show symptoms of CF. People born with only one CF gene (inherited from only one parent) and one normal gene are CF carriers. CF carriers do not show CF symptoms themselves, but can pass the problem CF gene to their children. Scientists estimate that about 12 million Americans are currently CF carriers. If two CF carriers have a child, there is a 1 in 4 chance that the child will have CF.
Almost 1,400 different mutations of the CF gene can lead to cystic fibrosis (some mutations cause milder symptoms than others). About 70% of people with CF have the disease because they inherited the mutant gene Delta F508 from both of their parents. This can be detected by genetic testing, which can be done in kids both before and after birth and in adults thinking about starting or enlarging their families.
Of all ethnic groups, Caucasians have the highest inherited risk for CF, and Asian Americans have the lowest. In the United States today, about 1 of every 3,600 Caucasian children is born with CF. This compares with 1 of every 17,000 African Americans and only 1 of every 90,000 Asian Americans. Although the chances of inherited risk may vary, CF has been described in every geographic area of the world among every ethnic population.
Scientists don't know exactly why the CF gene evolved in humans, but they have some evidence to show that it helped to protect earlier generations from the bacteria that cause cholera, a severe intestinal infection.
The diagnosis of CF is being made earlier and earlier, usually in infancy. However, about 15% of those with CF are diagnosed later in life (even adulthood). Symptoms involve the lungs and digestive organs and vary in severity.
A few kids with CF begin having symptoms at birth. Some are born with a condition called meconium ileus. Although all newborns have meconium — the thick, dark, putty-like substance that usually passes from the rectum in the first few days of life — in CF, the meconium can be too thick and sticky to pass and can completely block the intestines.
More commonly, though, babies born with CF don't gain weight as expected. They fail to thrive in spite of a normal diet and a good appetite. Mucus blocks the passageways of the pancreas and prevents pancreatic digestive juices from entering the intestines. Without these digestive juices, the intestines can't absorb fats and proteins completely, so nutrients pass out of the body unused rather than helping the body grow. Poor fat absorption makes the stools appear oily and bulky and increases the child's risk for deficiencies of the fat-soluble vitamins (vitamins A, D, E, and K). Unabsorbed fats may also cause excessive intestinal gas, an abnormally swollen belly, and abdominal pain or discomfort.
Because CF also affects epithelial cells in the skin's sweat glands, kids with CF may have a salty "frosting" on their skin or taste "salty" when their parents kiss them. They also may lose abnormally large amounts of body salt when they sweat on hot days.
Cystic fibrosis is the most common cause of pancreatic insufficiency in children, but a condition called Shwachman-Diamond Syndrome (SDS) is the second most common cause. SDS is a genetic condition that causes a reduced ability to digest food because digestive enzymes don't work properly. Some of the symptoms of SDS are similar to those of CF, so it may be confused with cystic fibrosis. However, in kids with SDS, the sweat test is normal.
Because CF produces thick mucus within the respiratory tract, kids with CF may suffer from nasal congestion, sinus problems, wheezing, and asthma-like symptoms. As CF symptoms progress, they can develop a chronic cough that produces globs of thick, heavy, discolored mucus. They also may suffer from repeated lung infections.
As chronic infections reduce lung function, the ability to breathe often decreases. A person with CF may eventually begin to feel short of breath, even when resting. Despite aggressive medical therapy, lung disease develops in nearly all patients with CF and is a common cause of disability and shortened life span.
By performing genetic tests during pregnancy, parents can now learn whether their unborn children may have CF. But even when genetic tests confirm CF, there's still no way to predict beforehand whether a specific child's CF symptoms will be severe or mild. Genetic testing also can be done on a child after birth, and can be performed on parents, siblings, and other relatives who are considering having a family.
After birth, the standard diagnostic test for CF is called the sweat test — an accurate, safe, and painless way to diagnose CF. In the sweat test, a small electric current is used to carry the chemical pilocarpine into the skin of the forearm. This stimulates sweat glands in the area to produce sweat. Over a period of 30 to 60 minutes, sweat is collected on filter paper or gauze and tested for chloride.
To diagnose CF, two sweat tests are generally performed in a lab accredited by the Cystic Fibrosis Foundation. A child must have a sweat chloride result of greater than 60 on two separate sweat tests to make the diagnosis of CF. Sweat test normal values for infants are lower.
Several other tests are standard parts of the routine care used to monitor a child's CF:
When kids are first diagnosed with CF, they may or may not have to spend some time in the hospital, depending on their condition. If they do, they'll have diagnostic tests, especially baseline measurements of their breathing (lung function) and a nutritional assessment.
Before they leave, their doctors will make sure that their lungs are clear and that they've started a diet with digestive enzymes and vitamins that will help them to gain weight normally. Afterward, they'll probably see their doctor for follow-up visits at least once every 1 to 3 months.
The basic daily care program varies from child to child, but usually includes pulmonary therapy (treatments to maintain lung function) and nutritional therapy (a high-calorie, high-fat diet with vitamin supplements). Kids with CF also can take oral doses of pancreatic enzymes to help them digest food better. They may occasionally need oral or inhaled antibiotics to treat lung infections and mucolytic medication (a mucus-thinning drug) to keep mucus fluid and flowing.
A new treatment for CF, which is still being researched, is an inhaled spray containing normal copies of the CF gene. These normal genes deliver the correct copy of the CF gene into the lungs of CF patients. Since 1993, more than 100 CF patients have been treated with CF gene therapy, and test trials are underway in at least nine different U.S. medical centers and other centers around the world. Another new therapy, called protein repair therapy, aims to repair the defective CFTR protein. Numerous medicines, including a spice called curcumin, are also being tested.
Caring for a child with CF can be tough at times, but parents need not feel alone. Doctors can usually refer them to a local support group linked to the Cystic Fibrosis Foundation.
Reviewed by: Floyd R. Livingston Jr., MD
Date reviewed: April 28, 2017