Some states screen all newborns for cystic fibrosis. Most children who have cystic fibrosis are diagnosed by the time they are 3 years old, although milder forms of the illness may not be diagnosed until the teen years. In diagnosing cystic fibrosis, a physician will perform a physical examination and review the medical history. If cystic fibrosis is suspected, various tests may be performed to confirm the diagnosis. These include:

• Sweat test. The sweat test is the most common diagnostic test for cystic fibrosis. A small amount of a chemical called pilocarpine is rubbed on the patient's arm or leg. An electrode that generates a small, painless electric current is placed on the spot to induce sweating. A gauze pad or filter paper is used to cover the spot for about 30 minutes. The sweat collected is then analyzed and the level of chloride (a chemical in salt) present is measured. High levels of chloride may indicate cystic fibrosis. The test cannot indicate the severity of cystic fibrosis.
  
  
• Lung function tests. These tests can reveal how much air the patient’s lungs can hold. Testing also can reveal how quickly air is breathed into and out of the lungs, and how efficiently the lungs add oxygen to and remove carbon dioxide from the lungs.
  
  
• Blood tests. A blood sample can be used in genetic testing to identify any abnormality of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which may indicate cystic fibrosis. Blood testing also can reveal other abnormalities associated with cystic fibrosis. In some states, blood testing is being used to test all newborns for cystic fibrosis.
  
  
• Fecal tests. A stool sample can reveal the presence of abnormal levels of fats, which people with cystic fibrosis typically cannot fully digest.
  
  
• Chest x-ray. This imaging test can reveal scarring resulting from the chronic inflammation associated with cystic fibrosis.
  
  
• Sinus x-ray. This imaging test can reveal inflammation of the sinuses, which is often associated with cystic fibrosis.
  
  
• Sputum cultures. A sample of mucus can be analyzed for the presence of bacteria growing within it. Bacterial infections are common in people with cystic fibrosis.

It is recommended that children with cystic fibrosis receive regular medical examinations at a healthcare facility that specializes in the disease. These facilities can also help with dietary and therapy questions, and serve as a resource for caregivers and patients.

Women who are pregnant can undergo prenatal testing that may reveal whether or not a fetus has cystic fibrosis. During an amniocentesis, a physician inserts a hollow needle through the abdominal wall of the uterus and obtains cells from the fluid surrounding the fetus. The fluid is analyzed to see whether the CFTR genes of the fetus are normal.

Chorionic villus sampling is another prenatal testing technique. Ultrasound is used to guide a thin tube into the uterus. A tiny piece of the placenta is then removed and analyzed. If the fetus has cystic fibrosis, the condition will be revealed in the cells of the placenta.

Finally, people can be tested before they have children to determine whether or not they are carriers of abnormal CFTR genes. A blood or saliva sample can allow examination of the CFTR gene. Such testing is about 90 percent accurate, according to the National Institutes of Health (NIH). However, there are hundreds of specific cystic fibrosis gene mutations, and not all of them are known. As a result, genetic testing does not detect everyone who is a carrier of cystic fibrosis. People who undergo genetic testing are advised to have genetic counseling.