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Friday, May 24, 2013
Can anyone please tell me how physicians/clinicians defined cystic fibrosis before the molecular nature was known?
Before the time of molecular diagnostic capabilities, a CF patient was diagnosed using a sweat chloride test. This measures the amount of chloride in the sweat and gives a value that can be diagnostic along with clinical findings of CF. I belive there is a very good description of this testing on line at http://www.cff.org/. To be diagnosed with CF a person must have either 2 positive sweat chloride tests completed at an acredited CF center along with the phenotypic features of CF (bronchiectasis, chronic sinusitis and failure to thrive, to name a few).
Currently, a patient can be diagnosed with the presence of 2 positive genes found at the molecular level. There continues to be a grey area where not all of these tests are conclusive. These inconclusive or "atypical" patients are usually followed at the CF center as well.
Additional information from Patricia Joseph, MD: Sweat chloride has been the gold standard for many years. This test relies on showing an increased concentration of chloride in sweat from patients with CF. Normal values are below 40 mg/dl. 40-60 is a “intermediate” zone and over 60 is consistent with CF. The test should have positive results on 2 separate occasions. This test is still used as the first test performed in patients suspected of having CF.
Today the diagnosis relies on having both of the following:
1. Clinical evidence of the disease. This can be lung or bowel abnormalities, failure to thrive, sinusitis and nasal polyps, but can be as simple as digital clubbing (a change in fingernails seen in several inflammatory diseases). Having a family member with CF included in this category as well.
2. Laboratory evidence of abnormal chloride exchange. This includes any of the following: a. Positive Sweat testing
b. Genetic studies showing the presence of 2 mutations shown to be associated with CF disease
c. Nasal Potential Difference studies that show a pattern consistent with CF. This test measures the electrical difference in the nose when different solutions that open and close ion channels are flushed over the cells. Patients with CF have a specific response that is different from normal. This is a specialized study only available in a few centers, used mostly for research.
Many states have recently implemented newborn screening. The tests used vary from state to state, but most use some combination of Immunoreactive trypsin (IRT- an enzyme that is found in higher-than-normal concentrations in the blood of newborn infants with CF) and genetics. The initial screening test requires further confirmatory testing to be certain the patient has CF.
Richard D Shell, MD
Clinical Associate Professor of Pediatrics
College of Medicine
The Ohio State University
Patricia Joseph, MS, MD
Associate Professor of Medicine
College of Medicine
University of Cincinnati