[The new reference equations of the Global Lung function Initiative (GLI) for pulmonary function tests]

Until recently, the reference equations available for pulmonary function tests (PFTs) have had several weaknesses: they have often been based on relatively weak samples of normal subjects; they used mathematical models that are not very efficient in describing the evolution of PFTs over age; there were different equations for children/adolescents and for adults; the expression of the results solely as a percentage of the predicted value did not provide a good indication of the statistical significance of any difference that may exist between a measured value and its reference value. The Global Lung Initiative (GLI) aimed to establish new reference equations for PFTs that do not have these disadvantages. Based on large, representative, reference populations and allowing individualization of homogeneous ethnic groups over a wide age range, the GLI uses a statistical model that does not have any a priori hypothesis regarding the evolution of PFTs as a function of age (these models therefore make it possible to describe, in a very precise manner, the PFTs over all age ranges). For a given PFT, the equation is the same regardless of age (no discontinuity on transition to adulthood). The GLI equations are used to define a reference value, a threshold value (lower limit of the normal) and a z-score that take into account age, sex, size and, for some PFTs, ethnicity. The reference equations of the GLI were established in 2012 for spirometry, in 2017 for the TLCO and will soon be established for lung volumes. Already the representation of ethnic groups not identified by the GLI and of subjects with extreme values of age and size is being questioned.