Introduction In patients with Idiopathic Pulmonary Fibrosis (IPF), forced vital capacity (FVC) is used classify disease severity and determine antifibrotic therapy. In patients unable to perform acceptable FVC manoeuvres, relaxed vital capacity (RVC) is often used interchangeably. It is unknown how the 2021 Global Lung Initiative (GLI) reference values for RVC compared to the 2012 GLI reference values for FVC, and how this may impact disease severity classification and antifibrotic prescription in patients with IPF.

Aims Compare the predicted value, % predicted and z-score for FVC and RVC between GLI 2012 and GLI 2021 reference values. To determine whether the choice of reference values impacts disease severity and antifibrotic therapy prescription in patients with IPF.

Methods Retrospective analysis of spirometry performed between September 2020 and May 2022 in patients with IPF. Predicted values generated using the online GLI calculator for FVC GLI 2012 and RVC GLI 2021.

Results The mean absolute predicted value for GLI 2012 was 3.77L Vs 4.06L for GLI 2021. GLI 2012 % predicted, and z-score was 71.3% and -1.85 (mild abnormality) respectively, while it was 66.3% and -2.71 (moderately severe abnormality) for the GLI 2021 reference values. 30% of patients had a FVC >80% using GLI 2012 compared to 21% of patients who had an RVC >80% using GLI 2021.

Conclusions In patients with IPF, the use of the GLI 2021 reference values tends to worsen disease severity classification and may impact antifibrotic prescription eligibility compared to the GLI 2012 reference values. Careful consideration should be made when using RVC interchangeably with FVC.