Changes in Spirometry Interpretative Strategies: Implications for Classifying COPD and Predicting Exacerbations

Background Recent guidelines for spirometry interpretation recommend both race-neutral reference equations and use of z-score thresholds to define severity of airflow obstruction. Research Question How does the transition from race-specific to race-neutral equations impact severity classifications for patients with COPD when using percent-predicted versus z-score thresholds, and do changes in severity correspond to clinical risk? Study Design and Methods This retrospective cohort study included Black and White patients with COPD and available spirometry from the Johns Hopkins Health System. Global Lung Function Initiative (GLI) 2012 (race-specific) and GLI-Global (race-neutral) equations were used to determine the Forced Expiratory Volume in 1 second percent-predicted and z-score values. Patients were classified as mild, moderate, or severe according to percent-predicted or z-score thresholds. Association between a change in severity classification from race-specific to race-neutral with COPD exacerbations and all-cause hospitalizations were evaluated using logistic regression. Results This cohort included 13,324 patients, of whom 9,232 (69.3%) were White (mean age: 65.7 years) and 4,092 (30.7%) Black (mean age: 61.1 years). More Black than White patients had a change in severity classification between approaches when using percent-predicted thresholds (20.2% vs 6.1%, p<0.001) but not with z-score thresholds (12.6% vs 12.3%, p=0.68). An increased severity classification under a race-neutral approach was associated with increased risk of exacerbation when using z-score thresholds (OR 2.34, 95% CI 1.51-3.63) but not when using percent-predicted thresholds (OR 1.08, 95% CI 0.61-1.93). A decreased severity classification under race-neutral was associated with lower risk of exacerbation with both percent-predicted (OR 0.49, 95% CI 0.28-0.87) and z-score (OR 0.67, 95% CI 0.50-0.90) thresholds. Interpretation The proportions of Black and White individuals reclassified were similar with z-score thresholds, and changes in severity corresponded to clinical risk with z-scores. These results support recent recommendations for use of race-neutral equations and z-score thresholds for spirometry interpretation.