Comparison of the "Fixed thresholds" and the "Z-Score" for Spirometric diagnosis of Asthma

Introduction and objective: The use of z-score to interpret lung function has been recommended by GLI 2012 [1] and ATS/ERS 2022 [2], with evidence of false positives in the diagnosis of asthma when using fixed thresholds of FEV1/FVC. The aim of our study is to analyze the pulmonary function tests (PFTs) of patients admitted to our explorations for suspected asthma according to the z-score and the fixed thresholds, with a comparison of the two methods.
Methods: Our study is retrospective, carried out over a one-year period, from February 2022 to February 2023.
Results: We collected 292 cases. The average age of patients was 30 years, with a clear predominance of women (65%).
A similarity of results according to the fixed thresholds and z-score was found in 235 cases (182 had a normal PFT and 53 had an OVD). An OVD was found in 39 cases according to the fixed thresholds, whereas PFT was normal according to the z-score. A reduction of FVC according to the z-score, while the PFT was normal to the fixed thresholds, was noted in 18 cases. It was related to a hyperinflation
In our study, using the z-score, we found 39 cases of false-positive spirometric diagnosis of asthma and under-diagnosis of reduction of FVC in 18 cases.
Conclusion: The prevalence of asthma based on spirometry is higher when using the fixed thresholds than when using the z-score, hence the need for standardization of interpretation methods, notably by GINA 2024.