Single breath nitrogen test as predictor of lung function decline and COPD over an 8-year follow-up

Background: The single breath nitrogen (SBN) test was proposed for early detection of "small airways disease" in the seventies. Few longitudinal studies have subsequently evaluated the relationships between SBN test measurements and lung function decline or COPD incidence. Aim: This study evaluates whether SBN test abnormalities may be significant predictors of lung function decline and COPD incidence over an 8-year follow-up. Study Design and Methods: In this longitudinal study, 907 adults (20+ years old; 56% males) from the prospective Po River Delta epidemiological study underwent SBN test at baseline and spirometry testing at both baseline and follow-up 8-year apart. Multinomial and multiple regression models were used to assess associations of SBN indexes and rates of FEV decline or risk of COPD incidence over time, after adjusting for sex, height and baseline age, FEV and smoking status. COPD was defined according to either GOLD or ATS-ERS criteria. Results: Among SBN indexes, only the slope of alveolar plateau (N-slope) was significantly associated with rates of FEV decline (7.93 mL/year for a one-unit change in N-slope, p<0.0001), and with an increased risk of developing COPD as defined by GOLD (RR 1.81, 95%CI 1.29-2.52, mild; RR 2.78, 95%CI 1.70-4.53, moderate or severe obstruction) and ATS-ERS criteria (RR 1.62, 95%CI 1.14-2.29, mild; RR 3.40, 95%CI 1.72-6.73, moderate or severe obstruction). Conclusion: In this population-based study, N-slope from SBN test is a significant predictor of lung function decline and COPD incidence over an 8-year follow-up, confirming the role of the "small airways disease" in the natural history of COPD.