Nasal potential measurements on the nasal floor and under the inferior turbinate: does it matter?

Measurement of nasal potential difference (NPD) is increasingly used as diagnostic test for cystic fibrosis (CF) and for in vivo evaluation of treatments aimed at correcting the defective function of the cystic fibrosis transmembrane regulator (CFTR) protein. Several methods are used to measure NPD. This study explores the influence of the site of measurement and compares NPD results obtained on the nasal floor and under the inferior turbinate.

NPD was measured in 34 CF, 26 heterozygote, and 61 control subjects. In every subject, measurements were taken simultaneously under the inferior turbinate in one nostril, and on the nasal floor in the other nostril. Criteria for interpretable tracings were predefined. Repeat measurements were done in 57 persons.

More interpretable tracings were obtained under the turbinate (120/124) than on the nasal floor (109/124), P = 0.015. Within each subject group, mean values obtained were similar for maximal basal potential, response to amiloride, and total chloride response. Both techniques discriminate well between CF and controls. Repeatability was similar with both methods: mean differences between two measurements approximated zero for most values. Also after correction for different number of interpretable tracings, simulation of sample size calculation for use in CFTR corrective trials was slightly in favor of measurements obtained on the nasal floor.

NPD measurements under the inferior turbinate and on the nasal floor have similar discriminative power for diagnostic use. Measurements under the turbinate result in a slightly higher proportion of interpretable tracings but sample size calculation slightly favors the nasal floor method.