Impact of Washout Duration to Account for Left Truncation in Register-based Epidemiologic Studies: Estimating the...

Incidence rates and cumulative incidences estimated using registers (e.g., electronic healthcare records) might be biased by including cases diagnosed before the inception of the register as being at risk. Washout periods can identify and exclude prevalent cases from analyses, but the impact of washout duration on estimates is unknown. We estimated risks of mental disorders according to different washout period durations.

This population-based cohort included all 6,478,162 individuals aged 1-80 years living in Denmark in 2010-2021. Using hospital contacts in 2010-2021, we estimated incidence rates and cumulative incidence of mental disorders according to different washout period durations (0, 1, 2, 5, 15, and 41 years) based on hospital contacts prior to 2010.

Without a washout period, the lifetime cumulative incidence of any mental disorder was 49.4% (95% confidence interval [CI]: 49.2%, 49.5%) for females and 45.1% (95% CI: 45.0%, 45.2%) for males. Estimates decreased when we increased the washout, reaching a lifetime incidence of 40.3% (95% CI: 40.1%, 40.4%) for females and 36.6% (95% CI: 36.5%, 36.8%) for males when using all available data (41 years of washout). Without a washout period, estimates for specific mental disorder types were up to 60% higher than those obtained with the maximum washout period, but the bias in absolute terms depended on the underlying risks.

While including all cases identifiable in a register may decrease uncertainty, the inclusion of prevalent cases as being at risk may lead to substantially overestimated measures. We highlight the need for caution when using administrative registers and electronic healthcare databases.