HOW IS CF DIAGNOSED?

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  • clinical signs
  • “sweat test”
  • amniocentesis (with genetic analysis)
  • chorionic villi sampling (CVS) (with genetic analysis)

Diagnosis: based on sweat test with above 60 mEq/L Cl- and either pancreas involvement or recurrent lung infections; genetic testing for mutations; screening of newborns possible with blood trypsin value (Chymotrypsin?). Some clinics (e.g., Yale) are said to routinely perform two sweat tests before confirming diagnosis.

Genetic CF Test

In the past, blood tests were required to screen for gene mutations because DNA is present in most cells. White blood cells lining the mouth carry DNA. Called Buccal Cell DNA collection, a cheekbrush (a very small wand) is used to brush off a few cells from inside of the cheek. (There is another test which consists of rinsing the cells from the mouth with a special mouthwash). The fragments of DNA supplied by the cheek brushing are then synthesized to match known sequences in the CFTR gene in a process known as PCR – Polymerase chain reaction. PCR permits enough copied DNA to be harvested. Confirmation of a normal sequence can be performed by a number of standard genetic analysis techniques.

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Is there is a genetic test that could tell me if I am a carrier?”

There is a test to determine if you are a carrier for several of the most common mutations that cause CF. There are something like four hundred mutations identified so far, though, so the test can tell you if you are a carrier, but if it comes back negative, you just know it’s unlikely.

Manifestations: sweat glands (high electrolytes); pancreatic disease (exocrine insufficiency, disruption of exocrine function); intestinal glands (meconium ileus); diabetes due to destroyed pancreas; liver disease (biliary cirrhosis, biliary tract obstruction); dehydrated mucus in the airways; lung infections (chronic bronchopulmonary infection); reduced lung function; infertility in male (vas deferens blocked), less in female; all these result in reduced life expectancy.

Great phenotypic variations: Research is exploring the relationship between phenotype and genotype. However, this is extremely difficult to do for a variety of reasons including the extremely large number of CF gene mutations, and a lack of understanding of how CFTR functions. To date correlation can be made about genotype and pancreatic (in)sufficiency (i.e., 15% of CFers are pancreatic sufficient resulting from multiple different mutations). Also, Richard Parad and colleagues from the Brigham and Women’s Hospital, Boston, presented a study at the 8th North American CF Conference (see Journal of Pediatric Pulmonology October, 1994) which found that patients with two copies of the CFTR Delta F508 mutation were five times more likely to be infected with mucoid pseudomonas aeruginosa compared to patients with only one copy (heterozygotes) of Delta F508.

Typically there are 25% higher rates of energy expenditure in CFers.

(12/97) The Gibson-Cooke method of sweat testing is considered by most clinics to be the most accurate. There are, like any test, some problems with sweat testing as a diagnostic method for CF, however it is the most reliable and often used method for diagnosing CF (genotyping needs to get a little better than the 70% accuracy rate showed by the patient registry database of the CFF).

The sweat test uses a chemical to called pilocarpine to produce sweat and then the sweat is collected and its sodium chloride rate is measured. The general rule of thumb is that levels (of sodium chloride) greater than 60 mEq/L are considered positive for cystic fibrosis. However, you will find this value differs minutely from clinic to clinic. There have been qualified diagnosis of CF made with readings of less than 60 mEq/L, however these are relatively rare.

There are other diseases with lung symptoms common to CF (but lack the GI – i.e. pancreatic insufficiency or liver disorders – common to CF). I have known a few people who have encountered problems with diagnosing whether or not they have CF. Some went on to an accurate (positive) diagnosis, while others remain in limbo but receiving treatment for lung symptoms. I understand your frustration ( I was diagnosed at the age of 24 after years of treatment for ‘bronchectasis’) and hope that you and your doctors are soon successful at teasing out a diagnosis in your case.

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