Cystic fibrosis (CF) is a genetic disorder caused by mutations in the cystic fibrosis transmembrane regulator (CFTR) gene. One of the main characteristic features of CF is the accumulation of thick, sticky mucus in different parts of the body, including the airways. This is caused by the abnormal transport of sodium and chloride ions across the epithelial cells lining the airways.
The transport of ions such as sodium and chloride creates voltage or electrical potential difference across the airway lining. This potential difference can be measured through the nose (known as the nasal potential difference, or NPD) by placing an electrode on the lining of the nose. NPD is used as an important diagnostic tool for CF. It also can be used to assess the effectiveness of different treatments.
How NPD is measured
NPD is measured using a device called a voltmeter, which records the potential difference across the epithelial membrane of the nose. Three electrodes are used for the measurement to be taken. One of these electrodes is placed on the surface of nasal mucosa or mucous membrane lining the nasal cavities. The second is placed under the inferior nasal turbinate (the portion of the nose that works to warm and moisten the air). The third electrode is placed under the skin as a reference.
After the electrodes are positioned, the lining of the nose is bathed with a series of solutions that contain different salts that are designed to change the flow of ions across the epithelium in predictable ways. Because patients with CF have dysfunctional CFTR chloride channels, the potential across their nasal epithelium responds differentially to the administration of these different solutions.
Advantages and limitations of NPD
NPD is an easily reproducible and minimally invasive technique that provides a direct measurement of the pathophysiological mechanisms of CF. The tolerance for this test is excellent and technical training to administer it is neither long nor costly.
However, there are some challenges and limitations to the use of NPD. These include:
- There is high variability in results between facilities. To ensure the quality and comparability of NPD, the CF Foundation Therapeutics Development Network (CFF-TDN) has developed standard operating procedures (SOP).
- The test is unreliable in case of inflammation or nasal infections such as sinusitis or acute rhinitis.
- Treatments such as inhaled therapies with hypertonic saline, antibiotics, amiloride, and DNase tend to normalize the potential difference in CF and make results unreliable.
- Repeated polyp removals and nasal scarring can alter the results or preclude the test from being performed.
- NPD measurements of the right and left nostril may be different. To overcome this discrepancy, it is recommended to take the best result rather than an average result.
- The presence of a large nasal polyp can obstruct visualization.
- NPD is useful for CF diagnosis in case of class 1 to 3 mutations only, but shows overlap values with healthy people if patients have classes 4 and 5 mutations. In such situations, other diagnostic tests need to be considered.
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