Reproducibility of nasal potential difference measurements in cystic fibrosis

Background: Nasal potential difference (NPD) measurement has been advocated as a diagnostic tool for cystic fibrosis (CF) patients and as a method for assessing the response to new therapies. The purpose of this study was to examine the reproducibility of NPD measurements performed in a single center. Methods: A total of 68 CF patients with a mean (± SD) age of 16 ± 8 years (age range, 6 to 52 years) underwent NPD measurements on at least two occasions. Results: A total of 25 patients with classic CF (mean age, 21 ± 8 years) and 43 patients with nonclassic CF (mean age, 14 ± 8 years) underwent sweat tests and NPD measurements. The mean sweat chloride values were 102 ± 18 and 54 ± 14 mEq/L, respectively, for classic CF and nonclassic CF groups. All patients underwent repeat NPD measurements. The basal NPD and the response to amiloride (ΔAmil) and response to Cl ^- free and isoproterenol (ΔCl^- free + iso) were very similar in both measurements. In the classic CF group, the basal potential difference values were -40 ± 12 vs -39 ± 11 mV (p = 0.57), respectively, for the first and second measurements; 27 ± 9 vs 26 ± 10 mV (p = 0.55), respectively, for ΔAmil; and 2.1 ± 3.8 vs 0.4 ± 2.9 mV (p = 0.07), respectively, for ΔCl^- free + iso. In the nonclassic CF group, the values were -32 ± 13 vs -28 ± 10 mV (p = 0.008), respectively; 19 ± 10 vs 17 ± 8 mV (p = 0.388), respectively; and -3.2 ± 4.6 vs -3.3 ± 4.4 mV (p = 0.876), respectively. Conclusion: When performed in a single center, NPD is a reproducible test for CF patients and thus may be a useful outcome measurement for assessment of the efficacy of new treatments.