Airflow Obstruction Categorization Methods and Mortality

Current guidelines recommend using forced expiratory volume in 1 second (FEV₁) % predicted to categorize the severity of airflow obstruction. There are limitations to using FEV₁ % predicted for this purpose, including bias associated with demographic factors and the inability to correct for "lung size." Other methods for grading the severity of airflow obstruction have been proposed to address these limitations.

Our objectives were to categorize airflow obstruction severity using these methods and then determine which method results in a categorization most closely associated with mortality.

Study subjects were patients aged 40-80 years tested in our pulmonary function test laboratories in the period 2002 to 2013 with airflow obstruction based on an FEV₁/forced vital capacity (FVC) less than the lower limit of normal. Categorization of airflow obstruction severity was determined using four methods: FEV₁ % predicted; FEV₁ % predicted adjusted by FVC % predicted; FEV₁/FVC confidence interval approach; and FEV₁ z-scores. Receiver operating characteristic curve analysis was used to determine which categorization method best predicts 5-year survival.

We identified 2,000 patients with airflow obstruction. Important differences in the categorization of airflow obstruction severity were observed using the different methods. More patients were categorized as having severe obstruction using FEV₁ % predicted and FEV₁ z-scores compared with FEV₁ % predicted adjusted by FVC % predicted and FEV₁/FVC confidence interval approach. FEV₁ % predicted was the best predictor of 5-year survival among the four methods studied.

In our study, categorizing airflow obstruction severity using FEV₁ % predicted best predicted 5-year survival. This validates the current guideline recommendation that FEV₁ % predicted be used to categorize the severity of airflow obstruction.