Cystic Fibrosis

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Diagnosis

Because CF is a genetic disease and researchers know which gene causes CF, one might think that the first and obvious way to find out if somebody has CF is to test their DNA for "the CF gene." The problem with this approach is that there are nearly 400 different variations of the CF gene, not all of which can be identified by DNA analysis. Instead, the diagnosis depends largely on a clinical test known as the sweat test, which provides a measure of Cl- (chloride ion) in the sweat and an indication of glandular dysfunction. Most, 98% to 99% of CF patients can be positively diagnosed using the sweat test.

Other diagnostics include:

• Prenatal diagnosis
• Medical history
• Family history
• Sweat Test
• Immunoreactive Trypsinogen Test
• Nasal potential difference measurement
• Genetic testing
• Pulmonary Function Test

Prenatal Diagnosis
Prenatal genetic testing can be used to determine whether an unborn baby has CF, although the testing is not foolproof, many of the nearly 400 different mutations that can lead to CF cannot be detected.

Because prenatal genetic testing is expensive and carries some risk to the mother, not all women who are CF carriers choose to have the tests done.

Prenatal testing involves either amniocentesis or a chorionic villus biopsy. In the former, cells are removed from the amniotic fluid – the fluid that surrounds the fetus. A chorionic villus biopsy involves sampling tissue that develops into the placenta.

Medical History
The medical history, beginning in infancy, is essential to diagnose CF. Several symptoms provide the physician with important clues (see Symptoms).

Family History
Because CF is an inherited disorder, any family history of CF suggests that the gene is present in the lineage.

Sweat Test
A sweat test, also known as a sweat chloride test or a sweat electrolyte test, provides a measure of the amount of salt in the sweat that comes from the skin and is the most common test for CF. It involves using a chemical (pilocarpine) and a mild electric current to make a part of the skin sweat, wrapping the area with plastic and a pad to absorb the sweat, and then collecting the sweat about one-half an hour later.

Immunoreactive Trypsinogen Test (IRT)
In newborn babies who cannot produce enough sweat for a sweat test, an IRT may be done. An IRT is a blood test that involves drawing blood a couple of days after birth and evaluating the presence of the protein trypsinogen. If the test is positive, it should be confirmed by a mutation analysis (i.e., genetic testing). The combination of an IRT and a mutation analysis is sensitive 90% to 100% of the time.

Nasal Potential Difference (NPD) Measurement
As Na+ (sodium) and Cl- ions move across the membranes of the cells lining the airway, they generate what is called an electric potential difference (the amount of energy required to move an electrical charge from one point to another). In the nasal passages, this electric potential difference is known as the nasal potential difference (NPD), and it can be easily measured with a surface electrode. Because Na+ and Cl- transport is abnormal in CF patients, NPD measurements are very different in CF patients than in people who do not have CF.

This test is especially helpful when the sweat electrolyte test and/or the genetic tests are inconclusive. However, the success of the test is highly dependent on the skill of the technician, and should be done in a special center.

Genetic Testing
A genetic test, also known as a genotype test or mutation analysis, is designed to analyze DNA for the presence of one of the several hundred mutations that can cause CF. The test involves collecting a sample of the patient's blood. The test cannot detect all of the mutations that can cause CF, however, so its sensitivity is only about 80% to 85%. Genetic testing cannot be used to predict the severity of symptoms. There is no way to know, based on a person's genotype, whether CF will be fatal or mild.

Generally, a genetic test is done if a patient's sweat test is negative and there is still high suspicion that the patient has CF.

Pulmonary Function Tests
Pulmonary function tests may be done to assess the patient's respiratory dysfunction and whether the patient is healthy enough to receive a lung transplant, if necessary.

Cystic Fibrosis, Diagnosis reprinted with permission from pulmonologychannel.com
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