Prevalence and incidence of inflammatory bowel disease in family members.

To determine the risk of having or developing inflammatory bowel disease (IBD) in a family member of an IBD patient, a population of 245 IBD probands was randomly selected from the University of Chicago IBD Registry and their family history was elucidated by questionnaire and follow-up telephone call. One hundred seventy-nine (73%) probands responded to the questionnaire. There were no demographic distinctions between those eligible for the study, those who were complete responders, those who were nonresponders, and those with a positive family history of IBD. Fifty-four family members from 40 proband families (22%) had confirmed IBD. Prevalence of IBD in family members at the time of diagnosis of the proband was highest for parents (4.6%), siblings (2.6%), and children (1.9%). Grandparents, aunts and uncles, and first cousins had prevalence of IBD of less than 1%. Incident case frequency was determined by dividing the number of cases incident after the diagnosis of the proband by all those ever at risk. The incident case frequency was highest for siblings (1.9%), parents (1.0%), and children (1.0%). There was concordance noted for type of disease in the proband and the relative. No association could be discerned between the familial risk of IBD and gender, race, or religion of the proband. Despite a high occurrence rate of proband families with IBD, the specific risk to first, second, or third degree family members is low.