American journal of respiratory and critical care medicine 2017 03 17() doi 10.1164/rccm.201611-2265OC
Global Lung Function Initiative recommends reporting lung function measures as z-score, and a classification of airflow limitation (AL) based on this parameter has recently been proposed.
To evaluate the prognostic capacity of the AL classifications based on z-score or percentage predicted in patients with chronic obstructive pulmonary disease (COPD).
A cohort of 2614 COPD patients recruited outside the hospital setting was examined after 57±13 months of follow-up, totalling 10,322 person-years. All-cause mortality was analysed, evaluating the predictive capacity of several AL staging systems.
MEASUREMENTS AND MAIN RESULTS
Based on GOLD guidelines, 461 patients (17.6%) had mild, 1452 (55.5%) moderate, 590 (22.6%) severe, and 111 (4.2%) very severe AL. According to z-score classification, 66.3% of patients remained with the same severity, while 23.7% worsened and 10.0% improved. Unlike other staging systems, patients with severe AL according to z-score had higher mortality than those with very severe AL (increase of risk by 5.2 and 3.9 times compared with mild AL, respectively). The predictive capacity for 5-year survival was slightly higher for FEV1 expressed as percentage of predicted than as z-score (areas under the curves: 0.714-0.760 versus 0.649-0.708, respectively). A severity-dependent relationship between AL grades by z-score and mortality was only detected in patients under the age of 60.
In COPD patients, the AL classification based on z-score predicts worse mortality than those based on percentage of predicted. It is possible that the z-score underestimates AL severity in patients over 60 years of age with severe functional impairment. .