Asthma affects around one in 30 people and contributes to nearly half a million deaths worldwide every year.1 It is estimated that asthma will cost the UK NHS £1.3 billion in 2023, including costs relating to primary care contacts and prescription of treatments for asthma.2

Treatments that aim to minimise risk of acute asthma exacerbations focus on reducing airway inflammation. However, despite significant advancements and increasing expenditure on asthma medications, incidences of asthma-related admissions to hospital and deaths have not improved over the last decade.3 This issue highlights the need for a more personalised ‘treatable traits’ approach4 whereby treatments are targeted and optimised based on an individual’s underlying disease mechanisms rather than diagnostic labels encompassing varying patterns of airway dysfunction and symptom expression, which may not correlate with levels of airway inflammation.5

Fractional exhaled nitric oxide (FeNO) is a non-invasive breath test that provides an objective measure of type-2 airway inflammation, which is typically steroid responsive. Higher FeNO levels have been shown to predict increased risk of acute exacerbations in patients with moderate-to-severe asthma.6 Risk of exacerbations is observed to be even higher when both FeNO and blood eosinophils are raised.6,7 It is proposed that, while FeNO reflects the degree to which blood eosinophils are attracted to the airways, the blood eosinophil count reflects the systemic pool of available eosinophils.8 These two components interact synergistically to increase exacerbation risk (Figure 1).9

FeNO and blood eosinophils in predicting risk of acute asthma exacerbations. High FeNO (50 ppb or higher) and high blood eosinophils (0.30 × 109/L or higher) are associated with increased risk of acute asthma exacerbations. FeNO is driven by IL-13 and reflects the degree to which blood eosinophils are attracted to the airways. Blood eosinophils …