Familial risk of inflammatory bowel disease: a population-based cohort study 1977-2011.

Objectives: Estimates of familial risk of inflammatory bowel diseases (IBDs), Crohn's disease (CD), and ulcerative colitis (UC) are needed for counseling of patients and could be used to target future prevention. We aimed to provide comprehensive population-based estimates of familial risk of IBD.

Methods: The study encompassed the entire Danish population during 1977-2011 (N=8,295,773; 200 million person-years). From national registries, we obtained information on diagnosis date of IBD (N=45,780) and family ties. Using Poisson regression, we estimated incidence rate ratios (IRRs) of IBD in relatives of IBD cases compared with individuals with relatives of the same type without IBD.

Results: The risk of CD was significantly increased in first-degree (IRR, 7.77; 95% confidence interval (CI), 7.05-8.56), second-degree (IRR, 2.44; 95% CI, 2.01-2.96), and third-degree relatives (IRR, 1.88; 95% CI, 1.30-2.71) to patients with CD, and was less pronounced in relatives to UC cases. Likewise, the risk of UC was increased in first-degree (IRR, 4.08; 95% CI, 3.81-4.38), second-degree (IRR, 1.85; 95% CI, 1.60-2.13), and third-degree relatives (IRR, 1.51; 95% CI, 1.07-2.12) of UC cases, and less pronounced in relatives of CD cases. IRRs increased with two or more IBD-affected relatives and were modified by age, with the highest family-related IRR observed in early life.

Conclusions: The risk of IBD is significantly increased in first -, second-, and third-degree relatives of IBD-affected cases, with up to 12% of all IBD cases being family cases. The risk is particularly pronounced in young individuals.