Impact of variable look‐back periods on the incidence rates of chronic diseases using real world data

Purpose Estimating disease incidence based on secondary data requires a look‐back period to exclude patients with pre‐existing disease from the incidence risk set. However, the optimal length of the look‐back period and its impact on incidence rates are often unknown. We studied the impact of the length of the look‐back period on incidence rates of 24 different chronic diseases. Methods Everyone residing in Sweden between January 1, 2005 and December 31, 2013 were identified from national registries and followed up to 2 years (through December 31, 2015). Outcome events were identified from inpatient and outpatient hospital contacts and incidence rates were calculated per 100 000 person‐years. The length of the look‐back period was varied with 6‐month increments, starting at 6 months. The maximum look‐back period of 9 years was used as reference period. Results There were 7 943 807 individuals with a look‐back period of at least 9 years (mean age 46.5 years) and a mean follow‐up time of 1.97 years. Incidence rates were higher across all diseases when restricting the look‐back to 1 year compared to 9 years, with a magnitude of overestimation of the incidence rates between 13% (temporal arteritis) and 174% (type 1 diabetes). However, for most diseases the effect of extending the look‐back period beyond 3‐5 years appeared comparably small. Conclusions This study illustrates how short look‐back periods cause overestimation of the incidence rates of chronic diseases, suggesting that sensitivity analyses with respect to look‐back period are considered, particularly using data sources with limited information on past medical history.