Diagnosis of Cystic Fibrosis

Doctors diagnose Cystic Fibrosis based upon results from several tests

Sweat Testing

This test is the standard for confirming a Cystic Fibrosis diagnosis because of its sensitivity and specificity, simplicity, and availability. 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.

Sweat Cl Concentration Ranges

Age	Normal (mmol/L)	Intermediate (mmol/L)	Abnormal (mmol/L)
≤ 6 mo	≤ 29	30–59	≥ 60
> 6 mo	≤ 39	40–59	≥ 60

Pancreatic Function Test

Pancreatic function should be assessed at the time of diagnosis, usually by measuring fecal fat excretion in stool within a period of 72 hours.

Pulmonary Chest X-ray

Chest x-rays are done at times of increasing pulmonary symptoms (difficulty breathing) by routinely checking every 1 to 2 years. X-rays define the extent of lung damage, airway abnormalities, hyperinflation (alveoli become less elastic in lungs) and bronchial wall thickening. 

On the left is a healthy lung x-ray, on the right is a x-ray with the diagnosis of Cystic Fibrosis

Pulmonary Function test

This is the best indicator of patient status and should be done routinely 4 times a year. Pulmonary function can now be evaluated in infants by using a raised volume rapid thoracoabdominal compression technique. Pulmonary function tests indicate hypoxemia with a expiratory flow between 25% and 75%. Fifty percent of patients experience improvement in pulmonary function after aerosol administration of a bronchodilator.

Also, screening oropharyngeal or sputum cultures should be done 4 times a year.

Newborn Screening Test

This is performed through a genetic test or a blood test. The genetic test shows whether a newborn has abnormal\mutated CFTR genes. The blood test shows whether a newborn's pancreas is working properly. Universal newborn screening for Cystic Fibrosis is now standard in the US; > 90% of cases are first identified by newborn screening, but about 10% are not diagnosed until adolescence or early adulthood. Despite advances in genetic testing, the sweat Cl test remains the standard for confirming a CF diagnosis in most cases because of its sensitivity and specificity, simplicity, and availability.

Carrier Screening Test

CF carrier screening is recommended for couples who are planning a pregnancy. If both parents carry a CFTR mutation, prenatal screening of the fetus can be done by amniocentesis (procedure where a small amount of amniotic fluid is withdrawn and tested).

http://felixonline.co.uk/science/2949/tackling-cystic-fibrosis/

http://www.merckmanuals.com/professional/pediatrics/cystic-fibrosis-cf/cystic-fibrosis