Background Exhaled nitric oxide (FeNO) and blood eosinophil count (B-eos) correlate with sputum eosinophil count in asthma and COPD, and cut-off values have been introduced to help decision making. However, these cut-off values have not been validated in large heterogeneous clinical cohorts. Our aim was to assess in a real life mixed airways disease population the abilities of currently recommended cut-off values of FeNO, blood eosinophil count and their combinations to predict the presence of airway inflammation as reflected by sputum eosinophil and neutrophil count. Methods We recruited 310 subjects with obstructive airway disease (260 with asthma,50 with COPD) from a tertiary referral centre. Induced sputum cell differentials, FeNO, B-eos and spirometry were measured. FeNO and B-eos were categorised as low (<25 ppb and <0.150 x 109 cells/L), intermediate (25–50 ppb and 0.15 – 0.29 x 109 cells/L) and high (>50 ppb and ≥0.3 x 109 cells/L), respectively. A composite variable of FeNO and B-eos was formed with four categories as follows: both high, either high, both low, and all other combinations. We assessed the ability of FeNO, B-eos and their composite to predict the presence of sputum eosinophilia (≥3%) and neutrophilia (>61%). Results The majority of subjects were on maintenance ICS (84.2%) and/or LABA (73.9%) and smaller proportions on LAMA (26.1%) and oral corticosteroids (16.1%). Both FeNO and B-eos were better in predicting sputum eosinophilia than in predicting neutrophilia. Having both FeNO and B-eos high was associated with an 80.4% probability of having sputum eosinophilia and a 25.5% probability of having sputum neutrophilia. On the other hand, having both FeNO and B-eos low was associated with a probability of only 2.9% of having sputum eosinophilia and a 61.8% probability of having sputum neutrophilia (Figure 1). B-eos performed equally well in subjects with asthma or COPD while FeNO performed better in subjects with asthma. Conclusion Currently recommended cut-off values of FeNO and B-eos have good ability to predict presence or absence of sputum eosinophilia in a mixed group of subjects with airways disease. These markers in combination also have a moderate ability to predict presence or absence of sputum neutrophilia.Abstract P13 Figure 1 Percentage of subjects having sputum eosinophils at least 3% in different categories of the composite variable of FeNO and B-eos