[Nasal potential difference test to diagnose cystic fibrosis]

Cystic fibrosis is usually diagnosed based on suspicion arising from a typical clinical picture and must be confirmed by either a finding of high chloride concentrations in sweat tests on 2 separate days or detection of 2 gene mutations. The nasal potential difference (NPD) test has been proposed to provide evidence of abnormal function of the cystic fibrosis transmembrane conductance regulator (CFTR), a receptor that forms a chloride ion channel. The test is especially useful for patients who have normal chloride concentrations in sweat tests and in whom 2 gene mutations related to cystic fibrosis have not been detected. The NPD test requires 2 electrodes connected to a voltmeter (a Tholy-Medicap device). One is placed on the nasal mucosa of the inferior turbinate and the other is placed subcutaneously on the forearm. A reading less than -40 mV is considered abnormal, as values under that cut point are never found in healthy individuals. Two abnormal NPD findings on separate days are required for a diagnosis of CFTR dysfunction. False negatives arise when the integrity of the epithelium is altered. After application of amiloride, NPD decreases more markedly in cystic fibrosis patients than in healthy individuals and applying isoproterenol or fenoterol after amiloride provokes no response in patients with the genetic defect that prevents chloride ion channel activation.