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Introduction The clinical utility of home fractional exhaled nitric oxide (FeNO) monitoring is unknown. The American Thoracic Society cut-point for an elevated FeNO in adults is >50 ppb and values above this threshold are associated with an increased risk of asthma exacerbations. A possible clinical application of home FeNO monitoring is it may facilitate many more FeNO measurements, without the constraints of attending a clinic appointment, and it is unknown how many more patients with a high FeNO may be identified with this approach compared to the usual practice of periodic clinic-based monitoring.Methods The INFORM ASTHMA trial was a 26-week, open-label randomised controlled trial that compared budesonide-formoterol versus terbutaline for symptom relief in 181 adults with mild-moderate asthma using maintenance budesonide. The primary outcome for the study was a clinic-based FeNO measurement 26 weeks after randomisation. As part of the study, participants were given a home FeNO measurement device, the ‘Vivatmo me’ and recorded a daily measurement for the first two weeks of the study followed by weekly measurements for 10 weeks. Clinic-based FeNO were measured at weeks 0 (randomisation), 13 and 26 using a ‘Vivatmo pro’ device. Agreement between participants with any home measurement >50 ppb and any clinic measurement were assessed by a two-by-two contingency table and associated McNemar’s test of agreement.Results The mean (SD) age of participants was 33.9 (15.5) years, with age at asthma diagnosis of 11.2 (11.1) years, the mean (SD) baseline ACQ was 1.5 (0.68). The median (IQR) baseline FeNO was 62.5 (36 to 100.5) ppb. The agreement is shown in the table 1. There were 154/181 (69%) participants with a FeNO >50ppb for any of the clinic-based measurements and 119 (66%) with a FeNO >50ppb with the home-based measurements; McNemar’s test for agreement was not statistically significant: Chi-square 1.1 (1df), P=0.30.Abstract S52 Table 1Agreement for FeNO >50ppb for home-based versus clinic-based measurement Any clinic measurement greater than 50 ppb N/181 (%) Any home measurement greater than 50 ppb No Yes No 48 (27) 14 (8) Yes 9 (5) 110 (67) Discussion In this study of home-based versus clinic-based measurements of FeNO, there was no evidence of disagreement in the proportions of participants who had at least one FeNO >50ppb. This is consistent with frequent home-based measurement not offering greater detection of high FeNO compared to more routine clinic-based measurement.