The recently published Global Lung Function Initiative (GLI) carbon monoxide transfer factor (T_LCO) reference equations provide an opportunity to adopt a current, all-age, widely applicable reference set. The aim of this study was to document the effect of changing to GLI from commonly utilised reference equations on the interpretation of T_LCO results.33 863 T_LCO results (48% female, 88% Caucasian, n=930 aged <18 years) from clinical pulmonary function laboratories within three Australian teaching hospitals were analysed. The lower limit of normal (LLN) and proportion of patients with a T_LCO below this value were calculated using GLI and other commonly used reference equations.The average T_LCO LLN for GLI was similar or lower than the other equations, with the largest difference seen for Crapo equations (median: −1.25, IQR: −1.64, −0.86 mmol·min⁻¹·kPa⁻¹). These differences resulted in altered rates of reduced T_LCO for GLI particularly for adults (+1.9% versus Miller to −27.6% versus Crapo), more so than for children (−0.8% versus Kim to −14.2% versus Cotes). For adults, the highest raw agreement for GLI was with Miller equations (94.7%), while for children it was with Kim equations (98.1%). Results were reclassified from abnormal to normal more frequently for younger adults, and for adult females, particularly when moving from Roca to GLI equations (30% of females versus 16% of males).The adoption of GLI T_LCO reference equations in adults will result in altered interpretation depending on the equations previously used and to a greater extent in adult females. The effect on interpretation in children is less significant.