Clin Chem Lab Med 2003; 41(1):61 – 67 © 2003 by Walter de Gruyter · Berlin · New York Teresinha Leal 1 , Jean Lebacq 2 , Patrick Lebecque 3 , Jean Cumps 4 and Pierre Wallemacq 1 * 1 Department of Clinical Chemistry, 2 Department of Physiology, 3 Department of Pneumology-Paediatrics, 4 Biostatistics, Université Catholique de Louvain, St Luc University Hospital, Brussels, Belgium Nasal potential difference (NPD) measurements have been proposed to assess defective ion transport in cys- tic fibrosis (CF). Implementing it routinely is, however, difficult. Therefore, a modified method based on nasal instillation in supine position at reduced flow rate was tested to evaluate its ability to discriminate CF from non-CF subjects. Classical and modified methods were compared in nine healthy subjects and there were no statistical differences. Following the new method, 97 tests were performed on 74 subjects divided in three cohorts: 21 CF patients and two control groups con- sisting of 19 patients with other pulmonary diseases and 34 healthy subjects. Twenty five children were en- rolled in this study. Maximal NPD in CF patients (– 44.9 ± 2.5 mV) was significantly different from that obtained in control groups (–18.1±1.6 and – 17.2 ± 1.1 mV). Depolarization after amiloride also dis- criminated CF patients (25.9 ± 1.4 mV) from control groups (10.5 ± 0.9 and 8.1 ± 0.7 mV). Marked repolariza- tion following isoprenaline plus amiloride in low chlo- ride solution was seen in control groups (– 15.7 ± 1.1 and –15.3±1.1 mV). We conclude that the modified method represents a simplified and equally effective approach to discriminate CF patients from non-CF sub- jects. Moreover, this method presents practical advan- tages for the patients related to hygiene and conve- nience, favoring its application in small children. Clin Chem Lab Med 2003; 41(1):61 – 67 Key words: Membrane potentials; Cystic fibrosis trans- membrane conductance regulator (CFTR); Cystic fibro- sis. Abbreviations: cAMP, cyclic adenosine monophos- phate; CF, cystic fibrosis; CFTR, CF transmembrane con- ductance regulator; NPD, nasal potential difference. Introduction Despite more than a decade of intensive investigation of the genetics, pathophysiology and clinical pheno- types of cystic fibrosis (CF), the disease remains one of the most common lethal hereditary disorders with au- tosomal recessive heredity in Caucasian population worldwide. The gene encodes the CF transmembrane conductance regulator (CFTR) (1), a 1480 amino acid, apical membrane glycoprotein that functions directly as a cyclic adenosine monophosphate (cAMP)-acti- vated chloride channel. The CFTR protein also modu- lates the activity of other cell membrane channels (2). It thus plays a key role in the movement of ions and wa- ter across epithelia. Mucous epithelia generate an electrical potential dif- ference between their two sides, the outside face being negative relative to the inside. This voltage is related to transport of Na + across the epithelium (3). Values for nasal transepithelial potential difference (NPD) in a healthy population are normally distributed around a mean value of – 20 mV (4). In CF patients, a hyperpolar- ization of the nasal membranes is observed: their NPD values are about double those of normal subjects. Pri- mary studies suggested that increased NPD could well discriminate CF patients from other populations (3), since patients suffering from other lung diseases, in- cluding bronchiectases, asthma and Young’s syn- drome (5) have NPD values similar to those recorded in normal subjects. However, later studies (6) have shown considerable overlap between the results of baseline NPD from CF and control populations. An optimal in vivo standardized protocol (7) has therefore been de- veloped to better discriminate CF from non-CF sub- jects. For the complete NPD test, a maximal voltage value is first determined; then the response of the nasal mucosa to superfusion of drugs and ionic solutions is assessed (amiloride, low-chloride content solution and finally isoprenaline, a cAMP elevating agent) (7, 8). A recent consensus report (9) includes, for the diagnosis of CF, in vivo evidence of a CFTR-mediated abnormality of ion transport across the nasal epithelium. Despite their widely recognized diagnostic role, mainly when the sweat test and/or routine DNA analysis are incon- clusive, and their eventual prognostic role (10, 11), NPD measurements are not yet widely used. This might be due to technical considerations such as insertion of the reference electrode in subcutaneous space (8), tech- nique for nasal superfusion (7), standardization of de- vices and presence of focal pathologies, e.g., viral in- fection (8, 12) and nasal polyps (13). The technique for nasal superfusion makes the test particularly difficult in small children and sedation is necessary. Even though some improvements appeared during the last years (12, 14, 15), a recent multicenter report (16) still empha- sizes high between-center variability, challenging the test as outcome measure. Improvement of the practical aspects and of the robustness of this test appears therefore mandatory to allow its wider use, for both di- agnostic and prognostic purposes, and for studies on *E-mail of the corresponding author: wallemacq@lbcm.ucl.ac.be Modified Method to Measure Nasal Potential Difference Brought to you by | Columbia University Library The Burke Library New York Authenticated | 172.16.1.226 Download Date | 8/8/12 2:11 PM