P11 How much does gas transfer add to diagnostic yield when spirometry is ‘normal’ using Bayesian and non-Bayesian approaches?

Rationale Previous evidence shows a 37.8% increase in diagnostic yield when gas transfer is used supplementary to a ‘normal’ spirometry based on conventional spirometry interpretation criteria. 1 However, in individuals with a higher pre-test probability of lung restriction, smaller deviations from the predicted forced vital capacity (FVC) may be considered pathological.Research question: How much value does gas transfer add, in addition to spirometry, when a Z score of -1.645 (ARTP) and -1 (Bayesian) are applied to classify the FVC?Methods Retrospective analysis of 560 patients confirmed to have ILD in the Birmingham Regional ILD multidisciplinary team (MDT) meeting was performed between 2015 and 2025. Evaluation of the frequencies and proportions of patients considered to have ‘normal’ lung function was performed using different cut-off thresholds. Lung function patters were split into normal, loss of volume and capillary structure (LVACS), extrapulmonary restriction (ER), and pulmonary vascular abnormality (PVA), based on ERS interpretive guidelines. 2 Results Spirometry was diagnostic in itself in 242 (43.2%) and 354 (63.2%) cases when Z scores of -1.645 and -1 were applied, respectively. The addition of gas transfer identified 268 (47%) and 161 (28.7%) additional abnormalities, supplementary to spirometry, when Z scores of -1.645 and -1 were applied. The sensitivity of FVC to predict a low VA increased from 61.2% to 85.9% when a FVC Z score of -1 is used instead of -1.645. See figure 1 for Sanke diagram of breathing test outcomes.Conclusion Use of a Bayesian approach to classifying FVC increases the diagnostic yield in patients with a high pre-test probability of ILD. LVACS (VA reduced, KCO normal or reduced) was the most common lung function pattern seen in the local ILD population. An FVC Z score of -1 is highly sensitive to a reduced VA in these individuals with high pre-test probability for ILD.Abstract P11 Figure 1References Sylvester, et al. Early respiratory diagnosis: benefits of enhanced lung function assessment. BMJ Open Respiratory Research 2021. Stanojevic. ERS/ATS technical standard on interpretive strategies for routine lung function tests. ERJ. 2022.