Background: Patients with irritable bowel syndrome (IBS) often report family members with similar symptoms, but family studies are lacking. We hypothesised that if there is familial aggregation, there would be an increased frequency of IBS in first degree relatives of IBS patients compared with relatives of controls (the patient's spouse). Methods: A valid self report bowel disease questionnaire (BDQ) that recorded symptoms, the somatic symptom checklist (a measure of somatisation), and a family information form (FIF) to collect the names and addresses of all first degree relatives were mailed to two groups of patients and their spouses (patients attending an IBS educational programme and residents of Olmsted County, Minnesota, who had been coded as IBS on a database). A BDQ was then mailed to all first degree relatives of subjects identified from the FIF. IBS diagnosis in the relatives was based on the Manning criteria. Results: The BDQ was sent to a total of 355 eligible relatives; 71% responded (73% relatives of patients, 67% relatives of spouses). Relatives were comparable in mean age, sex distribution, and somatisation score. IBS prevalence was 17% in patients' relatives versus 7% in spouses' relatives (odds ratio adjusted for age and sex 2.7 (95% confidence interval (CI) 1.2, 6.3)). When also adjusted for somatisation score, the odds ratio was reduced to 2.5 (95% CI 0.9, 6.7). Conclusions: Familial aggregation of IBS occurs, supporting a genetic or intrafamilial environment component, but this may be explained in part by familial aggregation of somatisation.
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