This is a medical question of type : exams and tests 
 How to diagnose Cystic Fibrosis ? 
 Doctors diagnose cystic fibrosis (CF) based on the results from various tests.
                
Newborn Screening
                
All States screen newborns for CF using a genetic test or a blood test. The genetic test shows whether a newborn has faulty CFTR genes. The blood test shows whether a newborn's pancreas is working properly.
                
Sweat Test
                
If a genetic test or blood test suggests CF, a doctor will confirm the diagnosis using a sweat test. This test is the most useful test for diagnosing CF. A sweat test measures the amount of salt in sweat.
                
For this test, the doctor triggers sweating on a small patch of skin on an arm or leg. He or she rubs the skin with a sweat-producing chemical and then uses an electrode to provide a mild electrical current. This may cause a tingling or warm feeling.
                
Sweat is collected on a pad or paper and then analyzed. The sweat test usually is done twice. High salt levels confirm a diagnosis of CF.
                
Other Tests
                
If you or your child has CF, your doctor may recommend other tests, such as:
                
Genetic tests to find out what type of CFTR defect is causing your CF.
                
A chest x ray. This test creates pictures of the structures in your chest, such as your heart, lungs, and blood vessels. A chest x ray can show whether your lungs are inflamed or scarred, or whether they trap air.
                
A sinus x ray. This test may show signs of sinusitis, a complication of CF.
                
Lung function tests. These tests measure how much air you can breathe in and out, how fast you can breathe air out, and how well your lungs deliver oxygen to your blood.
                
A sputum culture. For this test, your doctor will take a sample of your sputum (spit) to see whether bacteria are growing in it. If you have bacteria called mucoid Pseudomonas, you may have more advanced CF that needs aggressive treatment.
                
Prenatal Screening
                
If you're pregnant, prenatal genetic tests can show whether your fetus has CF. These tests include amniocentesis (AM-ne-o-sen-TE-sis) and chorionic villus (ko-re-ON-ik VIL-us) sampling (CVS).
                
In amniocentesis, your doctor inserts a hollow needle through your abdominal wall into your uterus. He or she removes a small amount of fluid from the sac around the baby. The fluid is tested to see whether both of the baby's CFTR genes are normal.
                
In CVS, your doctor threads a thin tube through the vagina and cervix to the placenta. The doctor removes a tissue sample from the placenta using gentle suction. The sample is tested to see whether the baby has CF.
                
Cystic Fibrosis Carrier Testing
                
People who have one normal CFTR gene and one faulty CFTR gene are CF carriers. CF carriers usually have no symptoms of CF and live normal lives. However, carriers can pass faulty CFTR genes on to their children.
                
If you have a family history of CF or a partner who has CF (or a family history of it) and you're planning a pregnancy, you may want to find out whether you're a CF carrier.
                
A genetics counselor can test a blood or saliva sample to find out whether you have a faulty CF gene. This type of testing can detect faulty CF genes in 9 out of 10 cases.