Does the single-breath N₂ test identify the smoker who will develop chronic airflow limitation?

We report here the results of a 9- to 11-yr follow-up of 2 cohorts in which spirometry and the single-breath N₂ test were used throughout the follow-up period to determine the usefulness of the single-breath N₂ test in identifying the smoker who is experiencing a rapid decline in FEV₁ and is therefore likely to be at risk of developing chronic airflow limitation. The analyses are based on 734 subjects tested from 3 to 5 times over the follow-up period; 82 smokers developed an abnormal FEV₁ during the follow-up period. Of these 71 (87%) had had an abnormal single-breath N₂ test at some time prior to the FEV₁ becoming abnormal. Of the single-breath N₂ test variables, CC/TLC was the only one significantly associated wtih the rate of decline of FEV₁ in both cohorts once adjustments were made for age, sex, height, and smoking. We conclude that the single-breath N₂ test can be useful in identifying the smoker who is at risk of developing chronic airflow limitation. However, its usefulness is diminished by the high proportion of smokers who have mild functional abnormalities but do not progress to develop chronic airflow limitation. We also find that the single-breath N₂ test does not appear to have a useful predictive value in nonsmokers.