Most individuals are diagnosed with cystic fibrosis before the age of two. Physicians who suspect cystic fibrosis usually order a sweat chloride test or a genetic test to confirm a cystic fibrosis diagnosis. A genetic test may be used when a child is too young to produce enough sweat for an accurate sweat chloride test.
Sweat Chloride Testing for Cystic Fibrosis
Diagnosing cystic fibrosis (CF) early is extremely important for the health of people with CF. Early intervention can help increase the life-span of individuals living with CF. The sweat chloride test has been the most reliable diagnostic tool used to diagnose cystic fibrosis for more than forty years. When the sweat test is performed correctly, and evaluated in an experienced laboratory, the sweat test is the most accurate test done today.
How is the Sweat Chloride Test Done?
The sweat chloride test uses a chemical to stimulate the sweat glands to produce sweat. The chemical is placed on an electrode and then placed on the forearm. The chemical reaches the sweat gland by an electrical current using a small hand-held device called iontophoresis.
The sweat is then absorbed by a special filter paper and sent to a laboratory for testing. The laboratory then analyzes the sweat for its sodium chloride (salt) content.
Understanding Sweat Chloride Levels
The normal sweat chloride values are 10-35 (mEq/L) milliequivalents per liter. A positive test is one in which the chloride (salt) value is 60 (mEq/L) milliequivalents per liter or higher. A negative test is one in which the chloride (salt) value is 40 (mEq/L) milliequivalents per liter or lower.
What if the Sweat Test is Between Normal and High Levels?
It is extremely rare for individuals to test in between 40 and 60 (mEq/L) milliequivalents per liter. If a sweat test does come back between 40 and 60 (mEq/L) milliequivalents per liter, the test should be repeated in the near future. It is also very rare to have a positive sweat test caused by a disease other than CF.
In severely malnourished individuals with CF, the sweat chloride level may be negative. However, once the malnutrition is corrected, the test may become positive.
Newborns
In addition, the sweat chloride test may not always be accurate in newborns. Infants do not produce enough sweat for a reliable diagnosis in the first month of life. The majority of doctors will wait until the infant is at least a few months old before giving the sweat test.
Source : American Medical Association. Fmaily Guide. 4th Edition. 2004. Cystic Fibrosisi Association.
