Using locally or regionally obtained normative data of FENO in children is very important when it comes to comparative studies and utilization of this clinical tool in clinical practice; therefore, we elected to study our local population. The number of subjects we studied was reasonably high, taking into consideration the previous studies of normal FENO level among children and the total population of the state of Qatar.
To minimize the effect of ethnic variation, only Qatari nationals were included in this study. Ethnic differences in FENO are well established in many studies.[1
] Buchvald et al
] showed that FENO is lower in Whites compared with the Blacks and Asians, although the number of non-whites was small, and this was confirmed by Kovesi et al
] The only published study for normative FENO values in the Arabian Gulf region is from Saudi Arabia for adult subjects.[20
] This study showed mean FENO levels of 22.79 ppb+/-8.13 which are comparable with the international figures, but higher than ours (14.1 ppb). The FENO level of our study population seemed to be higher than in whites[1
] but lower than in Asian children[19
] of the same age groups and using same measurement protocol . However, our values are closer to Caucasians than Africans and Asians. This is similar to the findings of Kovesi et al
. on children of Arabic descent (Personal Communication).
Figure 3 Age-related FENO values from different publications (Saito et al., Buchvald et al., Malmberg et al., Kovesiet al., Liet al.)
The cause of such ethnic differences is still not clear. It may be related to the rate of NO production by the alveolar or bronchial cells. Genetic studies among adults suggested an association between FENO and polymorphisms in the Nitric Oxide Synthase (NOS).[27
] In children, there is a recent evidence of an association between NOS polymorphism and atopy as well as asthma and FENO level.[25
] Another explanation for the ethnic difference in FENO levels may be related to the difference in the alveolar surface area that affect the amount of NO produced and hence exhaled. This may be explained by correlation of FENO with the weight, surface area,[1
] and pulmonary function test parameters[4
] in different studies among children. In our study, we did not show any statistically significant relationship between FENO and any of these variables.
Interestingly, gender difference in FENO was only found among Asians in most of the epidemiologic studies performed on children.[16
] We found similar gender difference among Qatari children; therefore, gender should be considered while interpreting FENO results among our population. Genetic polymorphism of NOS performed by Leung et al
] failed to explain this difference, which may indicate that the difference is due to hormonal or anatomic factors. The stimulatory effect of testosterone[33
] and the inhibitory effect of estrogen on FENO may indicate a hormonal cause of this gender difference, while higher pulmonary surface area among males[36
] may point toward anatomic effect.
Many studies found a positive correlation between FENO and increasing age among children,[1
] while others failed to prove such association.[16
] Unlike all previously published data, our study showed a negative rather than positive correlation of FENO with age. This is probably why FENO levels are higher than those reported in other studies for the younger age group (12-15 years) but lower for the older age group (16-18 years). In general, FENO levels tend to be higher in adults than in children.[18
] Whether this is due to the difference in lung size or increased endogenous production of NO related to NOS activity is still controversial. The first theory is supported by the finding that FENO level is affected by height and body surface area rather than age, both in adults and children.[4
] In addition, among children, a positive correlation was found between FENO and BMI.[1
] The other theory is supported by the notion that FENO continue to increase during adulthood despite fixed or even declining lung volume.[38
] However, in our study, no association was noticed between FENO and weight, height, BMI, or BSA using both univariate and multivariate regression analysis. Inadequate sample size to show such correlation is possible; however, our sample size was larger than some of other studies that revealed significant correlation.[4
] Therefore, FENO among our population may be affected by factors other than anthropometric measures.
The downward trend of FENO with age was interrupted by higher levels at age 15 years for boys, before the curve resumes its downward trend. A similar phenomenon was noticed among Asian children in a report by Kovesi et al.; in that study, there was a drop in FENO level at 9 to 10 years, while there was a rise at 10 to 12 year olds. In our study, we did not look for clinical or hormonal changes related to puberty, which is the possible mechanism. Testosterone surge at puberty may explain the increased level in our male subjects at 15 years. To the authors’ knowledge, there are no longitudinal cohort studies to monitor the FENO changes around puberty and correlate them to the hormonal changes.
Interestingly, there was no relationship between FENO level and cigarette smoke exposure, which was explained by NOS3 polymorphism difference which mediate cigarette effect on FENO in a previous study.[39
Among the environmental exposures, we included in the questionnaires that affect FENO level, only cat exposure had a significant negative effect on FENO level. Similar correlation had been reported by some studies;[40
] however, most of the others revealed opposite results.[39
] In another study of indoor factors affecting FENO, dogs were associated with a significant reduction in FENO, but cats were not.[42
] This may be explained by the hygiene hypothesis which correlates early animal exposure to reduced atopic propensity later in life. The same study observed, as we did, no relationship between environmental tobacco exposure and FENO.
We could not determine the sample size accurately without having a preliminary data to determine the approximate standard deviation of FENO level and prevalence of exclusion criteria, especially atopy, among our population. Moreover, the collection of samples took 5-month period and hence spanned two seasons, which may affect the results. However, such effect was not detected when data were stratified according to the month of sampling. The sample size targeted according to pre-study data to measure FENO reference values for the age range studied with 95% confidence was accomplished. However, given the standard deviation observed in the study and success rate of performing FENO, the sample size needed to determine reference FENO levels for each age group is estimated to be 45 subjects. Therefore, we intend to recruit 315 subjects in a follow-up study to achieve this aim for the 7 age groups.
In conclusion, the results of our study, which is the first for children in the region, revealed higher FENO levels than in Whites, but lower than Asians for the targeted age group with the same gender trend of Asians. Interestingly, we documented a previously unreported negative correlation of FENO with age, uprising level around puberty for males, and negative correlation with exposure to cats. Further studies in the region are needed to verify such associations. In addition, after the follow up study, we will formulate an equation for normative FENO levels for each age group in our population, taking in consideration the abovementioned variables that affect measured FENO values to use them in the daily clinical practice.