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BackgroundAccurate assessment of asthma symptom control is central to effective management. Spirometry is the gold standard for evaluating lung function and monitoring physiological impairment, and the Global Lung Function Initiative (GLI) has developed widely used spirometric reference equations applicable across populations. However, population-specific equations may better reflect regional characteristics. This study examined the impact of using different spirometric reference equations on asthma assessment.MethodsIn this cross-sectional study, pre-bronchodilator forced expiratory volume in one second (FEV₁), forced vital capacity (FVC), and FEV₁/FVC were assessed in 384 adults with asthma, and symptom control status was assessed using the Asthma Control Test (ACT). Reference values, z-scores, and lower limits of normal (LLN) were derived using the GLI Global (GLI-2022 race-neutral) equation (GLIgl), GLI-2012 Caucasian equation (GLI2012_cauc), GLI-2012 Other/mixed equation (GLI2012_other), and Al-Qerem local equation (LOCAL). Agreement between equations was evaluated using Cohen's kappa, and their sensitivity and specificity for identifying ACT-defined uncontrolled symptoms were assessed.ResultsSpirometric values differed significantly across equations. Associations between spirometric impairment and ACT-defined uncontrolled symptoms were modest, with low-to-moderate discrimination. Higher FEV₁ and FVC z-scores were consistently associated with higher ACT scores across all equations, although explained variance was limited and model-fit differences were minimal. Pairwise agreement between equation sets was substantial to near-perfect for FEV1 < LLN (kappa = 0.692-0.907) and FEV1/FVC < LLN (kappa = 0.757-0.945), while LLN-defined impairment rates differed across equations.ConclusionsThe choice of spirometric reference equation substantially influences impairment classification and its concordance with ACT-defined symptom control, but overall predictive performance remains modest. Spirometry should be interpreted as a complementary, rather than standalone, tool in asthma assessment, with careful consideration of population-appropriate reference standards.