How best to express the level of lung gas transfer (TLco) function has not been properly explored. We used the most recent clinical data from 13 829 patients (54% male, 10% non-European ancestry), median age 60.5 years (range 20-97), median survival 3.5 years (range 0-20) to determine how best to express TLco function in terms of its relation to survival. The proportion of subjects of non-European ancestry with Global Lung Function Initiative (GLI) TLco z-scores above predicted was reduced but was significantly increased between -1.5 to -3.5 suggesting the need for ethnicity appropriate equations. Applying GLI FVC ethnicity methodology to GLI TLco z-scores removed this ethnic bias and was used for all subsequent analysis. TLco z-scores using the GLI equations were compared with Miller's US equations with median TLco z-scores being -1.43 and -1.50 for GLI and Miller equations respectively (interquartile range -2.8 to -0.3 and -2.4 to -0.7, respectively). GLI TLco z-scores gave the best Cox regression model for predicting survival. A previously proposed six-tier grading system for level of lung function did not show much separation in survival risk in the less severe grades. A new four-tier grading based on z-scores of -1.645, -3 and -5 showed better separation of risk with hazard ratio for all-cause mortality of 2.0, 3.4 and 6.6 with increasing severity. Using GLI FVC ethnicity methodology to GLI TLco predictions removed ethnic bias and may be the best approach until relevant datasets are available.
Martin Raymond Miller, Brendan G Cooper