Transepithelial potential difference
The diagnosis of Cystic Fibrosis (CF) is usually based on high sweat chloride concentrations, characteristic clinical findings (including sinopulmonary infections), and/or family history. However, a small portion of patients with Cystic Fibrosis especially those with "mild" mutations of the Cystic Fibrosis Transmembrane Regulator ion channel (CFTR), have near-normal sweat tests.
In these cases, a useful diagnostic, adjunct involves measuring the nasal transepithelial potential difference (ie, the charge on the respiratory eithelial surface as compared to interstitial fluid). Individuals with Cystic fibrosis have a significantly more negative nasoepithelial surface than normal, due to increased luminal sodium absorption.
In most exocrine glands, the CFTR protein normally secretes chloride ions into the lumen, and also has a tonic inhibitory effect on the opening of the apical sodium channel (which absorbs sodium into the cell). Impaired CFTR functioning directly reduces ductal epithelial chloride secretion and indirectly increases sodium absorption through lack of CFTR's inhibitory effect on the apical sodium channel. The result is dehydrated mucus and a widened, negative transepithelial potential difference.
In the kidney, TEPD contributes to tubular reabsorption.