Asthma Is Not Associated With the Need for Surgery in Crohnʼs Disease When Controlling for Smoking Status: A Population-Based Cohort Study

Abstract

Introduction: There is growing evidence that asthma and Crohn's disease commonly co-occur. Despite the known associations between these two diseases, no previous studies have evaluated the impact of asthma on the risk of surgery in patients with Crohn's disease. Additionally, studies evaluating the risk of surgery in patients with Crohn's disease using health administrative data are generally limited by their ability to adjust for unmeasured confounding. The aim of our study was to assess the impact of asthma on the need for intestinal resection in Crohn's disease adjusting for smoking status, even though it was not measured in the main health administrative data, but was available in a smaller secondary dataset. Methods: Using health administrative data from a universally funded health care plan in the Western Canadian province of Alberta, we conducted a cohort study to assess the impact of asthma on the need for surgery in patients with Crohn's disease diagnosed between April 1, 2002 and March 31, 2008 (N=2,358). Validated algorithms were used to identify incident cases of Crohn's diseases and those with co-occurring asthma. Patients undergoing intestinal resection were identified using validated codes. The association between asthma and intestinal resection was estimated using Cox proportional hazards regression. Smoking status was imputed using a method based on Martingale Residuals that were estimated from a secondary dataset in which smoking status was measured. This second dataset included patients enrolled in the Alberta IBD Consortium between 2007 and 2014 who were chart reviewed and completed environmental questionnaires (N=509). All analyses were adjusted for age and sex. Results: Asthma did not increase the risk of surgery in either the health administrative data unadjusted for smoking status (HR 1.09, 95% CI 0.87 to 1.36) or in the secondary data adjusted for smoking status (HR 0.77, 95% CI 0.42 to 1.41) (Table 1). The association remained non-significant after using the secondary data to impute smoking status in the health administrative data (HR 0.98, 95% CI 0.80 to 1.20).Table: Table. Results of Cox Proportional Hazards Regression Models Using (1) Health Administrative Data from Alberta, Canada; (2) the Alberta IBD Consortium; and (3) Health Administrative Data with Smoking Status Imputed from the Alberta IBD ConsortiumConclusion: Although asthma is associated with an increased risk of IBD, co-occurring asthma is not associated with the risk of surgery in patients with IBD. This null association persisted after adjusting for smoking status. This study illustrates how to adjust for smoking status in health administrative data studies when it is only measured in a smaller secondary dataset.