Within-individual variation of spirometry measurements in primary care: a retrospective cohort study

Background: Forced expiratory volume in 1 s (FEV₁) and forced vital capacity (FVC) are two of the most common measurements in spirometry, which are used for the diagnosis and monitoring of respiratory disease. Within-individual variation in measured FEV₁ and FVC may affect their clinical utility but estimates of this are based on limited and often poor-quality data. 

Methods: A retrospective cohort study was performed. Data on FEV₁ and FVC results and sociodemographic, lifestyle and comorbidity covariates were extracted from the IQVIA Medical Research Database. A minimum of four measurements in the same individual within a 6-month time period from the first measurement was the only inclusion criterion. Within-individual measured variation was calculated as a coefficient of variation (CV) using a linear regression random effects model using within-subject variance to estimate CV for the whole population and various subgroups. 

Results: 4412 participants for FEV₁ and 3567 participants for FVC were included in the main study. The mean number of measurements per individual in the 6-month period was 4.8 (SD 2.3) for FEV₁ and 5.1 (SD 3.5) for FVC. The overall CV for FEV₁ was 22.4% (95% CI 22.1% to 22.8%) and for FVC was 15.2% (95% CI 15.0% to 15.5%). This is much higher than seen in a previous systematic review of spirometry variation. CV increased as mean patient FEV₁ and FVC decreased. 

Conclusions: Estimated within-individual variation in this analysis of real-world data is much higher than previously reported. Variation increases with more severe disease status. This has important implications for diagnosis, monitoring and clinical decision-making.