Question

A mom brings in a 6 y.o. child who has had recurrent sinusitis. You're worried about CF, so you order a "sweat test" (chloride sweat test), but this comes back normal. You next order a "nasal transepithelial potential difference" test. Why would a patient with CF have a significantly more negative nasoepithelial surface?

Answer 1

A patient with CF would have a significantly more negative nasoepithelial surface because the mutated CFTR gene causes defective channel proteins that don't effectively move Cl- ions, leading to a buildup of negative ions inside the cells and thick, sticky mucus that can result in infections.

Step-by-step explanation:

A nasal transepithelial potential difference test can indicate if a patient with cystic fibrosis (CF) has a more negative nasoepithelial surface because of alterations in ion transport. In healthy individuals, the CFTR protein functions as a membrane ion channel that transports Cl- (chloride ions) out of cells, maintaining a less negative charge on the outside. However, in CF, the mutated CFTR gene leads to defective channel proteins that don’t move chloride ions effectively. The lack of chloride ion transport means that Na+ (sodium ions) are not drawn to the extracellular space as they normally would be, resulting in less osmotic pressure to pull water into the mucus, making it thick and sticky.

This inability to move Cl- out of the cell causes an accumulation of negatively charged ions inside the cell, making the nasoepithelial surface significantly more negative. The mucus becomes dehydrated because the water does not move by osmosis to the area of higher ion concentration as it would in a normal situation. As a consequence, the mucus cannot be cleared easily by the cilia, causing respiratory and sinus infections and other symptoms of CF.