Diurnal variation of nasal nitric oxide levels in healthy subjects.

Nasal nitric oxide (nNO) has been suggested as a valuable marker of upper airway infl ammatory diseases, especially allergic rhinitis [1-3]. In order to interpret nNO values derived from single measurements, knowledge about a potential diurnal variation is mandatory. The existing data, however, do not yield a consistent picture [4-6]. The aim of this study was to assess the presence of diurnal variations of nNO levels in healthy subjects by closely spaced measurements. Measurements were performed in 19 healthy nonsmokers (12 female and 7 male). nNO determinations were done individually at fixed time points within 6 consecutive 2-hour periods between 7 AM and 7 PM. nNO was assessed with a chemiluminescence analyser (NOA 280, Sievers, Boulder, Colorado, USA) using a nasal olive in 1 nostril, with the other nostril open and a suction fl ow of 240 mL/min. Velum closure was confi rmed by oral exhalation against a resistance, and nNO was assessed when a plateau was reached. Measurements were made in triplicate for each nostril. In addition to paired t tests, repeated-measures analysis of variance (ANOVA) with Newman-Keuls post hoc comparisons was used for statistical analysis. There were no differences between the values obtained for each nostril; thus, the mean of these values was used. Comparison of the data assessed during each time period in each subject revealed statistically signifi cant changes over the course of the day (ANOVA, P < .0001). The mean (SEM) values for the 6 measurements repeated at 2-hour intervals between 7 AM and 7 PM were 1505 (113), 1650 (105), 1736 (109), 1730 (105), 1740 (95), and 1670 (103) parts per billion (Figure). Post hoc comparisons showed that the measurements made between 7 AM and 9 AM were signifi cantly different from those of all other time periods (P < .01), while according to pairwise t tests, measurements between 3 PM and 5 PM also differed signifi cantly from those between 5 PM and 7 PM (P = .019). These data indicate that nNO levels change over the day in healthy subjects. We did not have the opportunity to measure nNO during the night, but a lower nNO output during this period was recently described [7]. An early study found no difference in nNO over the course of the day [4]. However, consistent with our results, a steady increase has been described when comparing measurements performed from 8 AM 10 AM, 11:30 AM 1:30 PM, and 3 PM 5 PM [5], while another study found higher nNO between 1 PM and 3 PM compared to 7 AM 9 AM [6]. Performing a larger number of measurements, we observed an increase of nNO, starting with low morning values, reaching a plateau during the day, and then decreasing between 5 PM and 7 PM. Considering these fi ndings alongside existing data [5-7], a diurnal cycle of nNO with a minimum during the night and a maximum during the day could be postulated. This underlines the need for standardization of measurement time in clinical studies. The neglect of diurnal variation might be one reason for the inconsistency of data in allergic rhinitis compared to healthy controls, as some studies showed elevated nNO in allergic rhinitis but others similar levels [1]. Further studies should assess diurnal variation in subjects with diseases