Background: Patient symptoms, spirometry measurements, exacerbation rates, and exhaled nitric oxide (FE(NO)) levels have all been used to quantify asthma severity.
Objective: To determine the relationships among these disease surrogates in clinical practice.
Methods: Data were collected from 5 primary care asthma clinics on patient symptoms, reliever use, spirometry measurements, maintenance pharmacotherapy, disease severity (British Thoracic Society treatment step), and FE(NO) level. Exacerbation data (asthma-related unscheduled health care contact or rescue oral corticosteroid therapy) for the 12 months before and 3 months after the clinic visit were then obtained.
Results: A total of 267 adult asthmatic patients (mean [SEM] age, 51.6 [1.1] years; forced expiratory volume in 1 second, 86.3% [1.2%] of predicted) participated, and 157 exacerbations were captured. For the 12 months before the clinic visit, exacerbation rate was positively correlated with dose of inhaled corticosteroid (P < .001), treatment step (P < .001), reliever use (P = .002), and symptom score (P < .001) but was negatively correlated with FE(NO) level (P = .04); only symptom scores correlated with exacerbation rate in the 3 months after the visit. Levels of FE(NO) were significantly lower in frequently exacerbating patients receiving higher doses of maintenance inhaled corticosteroids compared with patients with mild disease who were corticosteroid naive (19.7 vs 40.4 ppb, P < .001). Measurement of FE(NO) was an insensitive method (sensitivity, 66.7%; specificity, 51.9% at a cutoff value of 20 ppb) for identifying patients who subsequently exacerbated.
Conclusion: Levels of FE(NO) are paradoxically decreased in patients with more severe asthma and frequent exacerbations and may, therefore, be of limited utility in primary care.