Applicability of the Global Lung Function Initiative-2022 spirometry reference equations among healthy adults in Iraq

The Global Lung Function Initiative (GLI) has developed race-neutral reference equations to represent global diversity better and improve fairness across populations. However, the applicability of these equations across different populations, including Middle Eastern populations, remains to be explored. This study aimed to evaluate the suitability of the GLI-2022 equations among healthy Iraqi adults. A cross-sectional study was conducted on 3,828 healthy non-smoking adults (49.8% females) who were 18 years and older. Spirometric measurements, including forced vital capacity (FVC), forced expiratory volume in one second (FEV₁), and FEV₁/FVC were obtained, and z-scores were calculated using the GLI-2022 equations. Mean z-scores, age and height were assessed using linear regression and quantile analyses. Mean z-scores for all spirometric measurements were above zero in both sexes. Among females, mean z-scores for FEV₁, FVC and FEV₁/FVC were 1.18 (SD 1.41), 0.81 (SD 1.29) and 0.666 (SD 0.92) respectively. In males, the corresponding values were 0.96 (SD 1.41), 0.61 (SD 1.30) and 0.643 (SD 1.02), with p < 0.001 suggesting that the GLI-2022 equations systematically underestimated lung function. None of the mean z-scores fell within the physiologically acceptable range (±0.5), indicating a poor fit. Additionally, z-scores varied significantly with age and height. Taller and older individuals showed lower FEV₁ and FVC z-scores, indicating an overestimation of lung function. This variability was more pronounced in males. The GLI-2022 equations systematically underestimated FEV₁ and FVC of the Iraqi population, with mean z-scores exceeding the acceptable ±0.5 range. Age- and height-dependent deviations further indicate that GLI-2022 does not adequately represent this population, underscoring the need for regional validation before adopting these equations to ensure accurate diagnosis and appropriate clinical management.