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Poster presentations: Epidemiology 2020

P740 Age determines the risk of familial IBD: A nationwide case–control study

J. Halfvarson1, J. Ludvigsson2, M. Sachs3, J. Askling3, O. Olén3

1Department of Gastroenterology- Faculty of Medicine and Health, Örebro University, Örebro, Sweden, 2Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden, 3Clinical Epidemiology Division- Department of Medicine Solna, Karolinska Institutet, Stockholm, Sweden

Background

A family history of inflammatory bowel disease (IBD) is a proxy for an individual’s genetic and, in part, environmental risk of developing IBD, and is one of the strongest known risk factors for developing IBD. Although a familial aggregation is a well-established concept, data on its strength and nature are sparse. Therefore, to provide precise estimates of the familial aggregation, we performed a nation-wide, case-control study and examined the relative risk for patients with IBD (vs. population controls) to have a first-degree relative (FDR) with IBD, by age of onset, type of family history, and IBD subtype.

Methods

Individuals diagnosed with IBD between 1 January 2003 and 31 December 2017 with at least one first degree relative (FDR) were identified from Swedish national registers and compared with non-IBD controls, with at least one FDR, from the general population. We used logistic regression to calculate odds ratios (ORs) as estimates of relative risk for IBD in FDRs taking age at IBD onset, sex, county, and year of IBD diagnosis into account.

Results

We identified 50,667 incident cases with IBD (CD, n = 14,927; UC, n = 27,221 and IBD-U, n = 8519) and 506,720 controls, matched for age, sex, birth year and county of residence. The overall familial relative risk estimate of IBD for siblings (OR 3.6 [95% CI 3.4–3.8]) was similar to that between generations (OR 3.5 [95% CI 3.3–3.7] for mothers and OR 3.5 [95% CI 3.3–3.7] for fathers). Half-siblings conferred a lower yet elevated relative risk (OR 2.0 [95% 1.8–2.2]). The familial relative risk estimates of IBD increased with number of affected FDRs (Figure 1). Pronounced differences were seen between subtypes of IBD in the index case and also across age of onset in the index case and in the FDR (Figure 2). Lowest odds ratios were seen for having an FDR with elderly onset IBD among cases who were diagnosed with elderly onset Crohn’s disease (OR 1.9 [95% CI 1.4–2.5]) or ulcerative colitis (OR 1.9 [95% CI 1.5–2.3]). Highest odds ratios were seen for having an FDR with paediatric onset IBD among cases who were diagnosed with paediatric Crohn’s disease (OR 10.6 [95% CI 8.2–13.5]) or paediatric ulcerative colitis (OR 8.4 [95% CI 6.4–10.9]).

Conclusion

The relative risk of familial IBD varies by IBD subtype as well as by age of onset in the index patient with IBD and the FDR, pointing to differences in genetic predisposition and environmental risks between different phenotypes of IBD.