A measurement comparability study to support instrument harmonization in the Canadian longitudinal study on aging

Longitudinal studies are essential for understanding health trajectories, but necessary upgrades to measurement instruments over time may compromise data comparability. The Canadian Longitudinal Study on Aging conducted a measurement comparability study to quantify systematic differences between instruments and derive conversion equations to correct for any differences. Overall, 50 participants aged ≥ 45 years (52% females) underwent repeated assessments with old and new instruments including the blood pressure monitor, electrocardiogram (ECG), carotid ultrasound, spirometer, audiometer, tonometer, and dynamometer. Linear mixed-effects models were used to identify systematic differences. Cohen's d was used to assess standardized mean difference, Bland-Altman plots evaluated agreement between instruments, and repeatability was assessed within instruments. Mixed-effects models showed no clinically meaningful difference across instruments, except for systolic blood pressure (SBP), which was on average 5.19 mmHg (95% CI, 2.64, 7.74) higher using the new instrument. Cohen's d demonstrated moderate systematic differences for several measures. Repeatability estimates showed good-to-excellent reliability for most measures; however, certain ECG and tonometer parameters showed moderate or poor precision. The results support the utility of conversion equations to account for systematic differences between instruments. The difference in SBP warrants the implementation of a conversion equation, whereas it is unclear for measures with moderate differences.