Impact of left censoring on longitudinal analyses of dementia using electronic health record data

Longitudinal electronic health record (EHR) data provides an opportunity to evaluate associations between certain risk factors and dementia incidence. In practice, researchers using EHR usually extract data for multiple years and use the first 2‐4 years as a washout period to exclude baseline prevalent cases. However, identifying dementia via clinical diagnostic codes during a washout period likely under‐detect some cases, with time to dementia diagnosis for these cases left censored and estimates for the associations of interest potentially biased. This study aims to assess the amount of left censoring when analyzing time to dementia using EHR data from the Indian Health Service (IHS).