Challenges and Pitfalls of Using Repeat Spirometry Recordings in Routine Primary Care Data to Measure FEV(1) Decline in a COPD Population

BACKGROUND: Electronic healthcare records (EHR) are increasingly used in epidemiological studies but are often viewed as lacking quality compared to randomised control trials and prospective cohorts. Studies of patients with chronic obstructive pulmonary disease (COPD) often use the rate of forced expiratory volume in 1 second (FEV(1)) decline as an outcome; however, its definition and robustness in EHR have not been investigated. We aimed to investigate how the rate of FEV(1) decline differs by the criteria used in an EHR database. METHODS: Clinical Practice Research Datalink and Hospital Episode Statistics were used. Patient populations were defined using 8 sets of criteria around repeated FEV(1) measurements. At a minimum, patients had a diagnosis of COPD, were ≥35 years old, were current or ex-smokers, and had data recorded from 2004. FEV(1) measurements recorded during follow-up were identified. Thereafter, eight populations were defined based on criteria around: i) the exclusion of patients or individual measurements with potential measurement error; ii) minimum number of FEV(1) measurements; iii) minimum time interval between measurements; iv) specific timing of measurements; v) minimum follow-up time; and vi) the use of linked data. For each population, the rate of FEV(1) decline was estimated using mixed linear regression. RESULTS: For 7/8 patient populations, rates of FEV(1) decline (age and sex adjusted) were similar and ranged from −18.7mL/year (95% CI −19.2 to −18.2) to −16.5mL/year (95% CI −17.3 to −15.7). Rates of FEV(1) decline in populations that excluded patients with potential measurement error ranged from −79.4mL/year (95% CI −80.7 to −78.2) to −46.8mL/year (95% CI −47.6 to −46.0). CONCLUSION: FEV(1) decline remained similar in a COPD population regardless of number of FEV(1) measurements, time intervals between measurements, follow-up period, exclusion of specific FEV(1) measurements, and linkage to HES. However, exclusion of individuals with questionable data led to selection bias and faster rates of decline.