DNA Direct: Your Genes in Context

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Testing for Genetic Disorders

Related Genetic Tests

Cystic Fibrosis

Includes a personalized report, a letter for your doctor, and toll-free support

Price: $260.00

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Pre-test & Post-test Consultation

Our genetic experts can counsel you before and after testing.

Price: $250 per hour

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CYSTIC FIBROSIS

Diagnosing Cystic Fibrosis

The "gold standard" for diagnosing someone with cystic fibrosis (CF) is the sweat test. It is a simple, non-invasive test that measures the amount of salt (sodium chloride) in the sweat. The sweat test is not able to identify CF carriers.

Genetic changes in the CFTR gene affect the salt balance in the body, so people with CF have higher levels of salt in their sweat. A few people with symptoms of CF (1 to 2%) have a borderline or even a normal sweat test. In these cases, genetic testing is recommended to confirm diagnosis.

The Sweat Test
The Genetic Test
Genetic Sequencing

The Sweat Test

Diagnosing CF usually requires two or more sweat tests in which chloride levels are positive for CF. The sweat test is not an effective test for identifying CF carriers. Carriers need genetic testing for accurate information.

The sweat test is conducted by a doctor. A colorless, odorless chemical is placed on the skin, which causes it to sweat, and a device collects the sweat. A sweat test can take 30 minutes to 1 hour, depending on how long it takes to collect the sweat.

Sweat Chloride Test Results
< 40 =	Negative for CF
40–59 =	Borderline
> 60 =	Positive for CF

If a person is diagnosed with CF by the sweat test, genetic testing helps doctors identify the underlying genetic mutations that are causing the CF. This can help family members determine their risk and carrier status. In certain cases, this can help the person with CF with their prognosis.

The Genetic Test

Genetic testing can tell whether a person has CF (two mutations), is a CF carrier (one mutation), or does not have any CF mutations.

Most labs currently offer genetic testing that looks for 32 of the most common mutations associated with CF. These mutations account for around 90 percent of all CF mutations. This means that testing does not identify all possible mutations. If a person has signs and symptoms of CF but is only found to have one mutation, then chances are that he or she has another, rare genetic change that cannot be identified by testing.

There are two ways to do genetic testing for CF. One involves taking a sample of DNA using a cheek swab that you brush against the inside of your cheeks to collect cheek cells. (This is what DNA Direct offers.) Other labs use blood samples to look at the same DNA.

Genetic Sequencing

The CF gene (CFTR) is very large. One type of genetic testing, called sequencing, looks at the entire gene and can detect almost all changes or alterations in it. Genetic sequencing, however, is not the best or most efficient test in most situations.

The problem with genetic sequencing for CF is that researchers have identified over 1000 mutations or variations in this gene. So, even if sequencing finds a genetic change, doctors and scientists may not be able to interpret whether that change is a normal variation or whether it is a disease-causing variation.

For this reason, most labs offer genetic testing only for the most common mutations known to be related to CF.