1. Included studies definitions.
| Study ID | Exacerbation definition | FeNO cutoff (ppb) | Symptom strategy | Duration |
| Calhoun 2012 | Increased asthma symptoms resulting in use of oral corticosteroids, increased ICS, or additional asthma medications | < 22 decrease 22 to 35 maintain > 35 increase |
Based on National Heart, Lung, and Blood Institute guidelines | 9 months |
| Hashimoto 2011 | Decrease in morning FEV1 > 10% compared to mean FEV1 from week before, increase in symptoms requiring increased prednisolone > 10 mg/day, or course of antibiotics, regardless of hospitalisations | > +10 ppb & > 10% increase > +10 ppb & ≤ 10% maintain minus ≥10 & ≤ 10 maintain < ‐10 ppb decrease |
Based on Global Initiative for Asthma (GINA) guidelines for treatment of severe asthma | 6 months |
| Honkoop 2014 | Severe exacerbations defined as hospitalisation, emergency department visit because of asthma, or use of oral corticosteroids for 3 or more days | < 25 decrease 25 to 50 no change > 50 increase |
Asthma Control Test, 7 items which includes lung function | 12 months |
| Powell 2011 | Events for which the participant sought medical attention (unscheduled doctor visit, emergency department visit, hospital admission, or when oral corticosteroids were used to treat increase in asthma symptoms) | < 16 decrease 16 to 29 maintain > 29 increase |
Asthma Control Test | 4 months |
| Shaw 2007 | Increasing asthma symptoms requiring course of oral steroids or antibiotics | < 16 once or 16 to 26 second decrease > 26 increase |
BTS and Asthma Control Test | 12 months |
| Smith 2005 | Minor exacerbation: global daily asthma score of 2 on ≥ 2 consecutive days Major exacerbation: global daily asthma score of 3 on ≥ 2 consecutive days |
< 15 maintain ≥ 15 increase (250 mL/sec) |
GINA guidelines | 12 months (Phase 2) |
| Syk 2013 | Increasing symptoms requiring a course of oral corticosteroids | Women < 19 decrease step 19 to 23 maintain ≥ 24 increase 1 step ≥ 30 increase 2 steps Men < 21 decrease step 21 to 25 maintain ≥ 26 increase 1 step ≥ 32 increase 2 steps |
Based on patient‐reported symptoms, SABA use, physical examination, and spirometry results | 12 months |
FeNO: fractional exhaled nitric oxide FEV1: forced expiratory volume in 1 second ICS: inhaled corticosteroids SABA: short‐acting beta‐agonist