Assessment of respiratory function: influence of spirometry reference values and normality criteria selection

Area of Science:

• Pulmonary Function Testing
• Clinical Diagnostics
• Respiratory Medicine

Background:

• Accurate spirometry is crucial for diagnosing and managing respiratory diseases.
• Patient classification relies on comparing measured lung function to predicted values.
• Existing spirometry reference equations and lower limit of normal criteria vary, potentially affecting clinical decisions.

Purpose of the Study:

• To assess the impact of different spirometry reference equations on patient classification.
• To evaluate the effect of varying lower limit of normal criteria on diagnostic outcomes.
• To highlight the importance of selecting appropriate predictive equations and thresholds in clinical practice.

Main Methods:

• Analyzed 1000 consecutive spirometries.
• Calculated predicted forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) using equations from Morris, Cherniack, Crapo, Knudson, and ECCS.
• Quantified classification changes based on equation choice and lower limit criteria (90% confidence interval vs. 80% of predicted).

Main Results:

• Significant differences in predicted values were observed between equations, particularly for FEV1 and FVC.
• Up to 35% of tests changed classification from normal to abnormal for female FEV1 when using 80% of predicted and different equations.
• A high percentage of tests had lower limits of normal below 80% and 70% of predicted values when using confidence intervals.

Conclusions:

• The choice of reference equation and lower limit criterion substantially influences spirometry-based patient classification.
• Using a fixed percentage of predicted value as the lower limit of normal is not recommended due to potential misclassification.
• Standardizing reference equations and lower limit criteria is essential for consistent and accurate clinical interpretation of spirometry results.