From a different angle: Diagnostic Performance of the Beta Angle in Idiopathic Pulmonary Fibrosis versus Chronic Hypersensitivity Pneumonitis – A Cross-Sectional Study

<bold>Introduction:</bold> Discrimination between Idiopathic Pulmonary Fibrosis (IPF) and Hypersensitivity Pneumonitis (HP) can be challenging. An obstructive airflow component may be present in HP, both in imaging and on clinical examination. The Beta Angle (BA) is a geometric index quantifying the concavity level of the spirometric expiratory flow/volume curve, allowing assessment of obstructive involvement in the small airways. This study aimed to compare BA values between HP and IPF patients. <bold>Methods:</bold> Cross-sectional study. Patients over 18 years of age, diagnosed with HP or IPF between January 1, 2018, and December 31, 2022, at a tertiary care center were included. BA was measured in stable patients with blinding to diagnostic status. Wilcoxon rank-sum test was used to compare the main outcome variable between groups. The association of clinical and tomography findings to BA was assessed through bivariate analysis and logistic regression with multivariate analysis. <bold>Results:</bold> 136 patients were evaluated, 77 with HP and 59 with IPF. No significant differences were found in BA between the two groups when analyzing the overall population. However, a statistically significant difference between groups (p=0.04) was identified when evaluating the subgroup of patients with a Forced Vital Capacity (FVC) greater than or equal to 75%. Additionally, a higher amount of air trapping on tomography was associated with a BA of less than 175 degrees (p=0.03) in an univariate analysis. <bold>Conclusion:</bold> BA measurement is a non-invasive tool that adds information to the evaluation of suspected IPF or HP in the early stages of the disease.